Why does lightning generate multiple branches? Everyone have seen and knows how lightning occurs. But then I realized that lightning never occurs in straight lines but it follows branching. But Why is that so? Why can't lightning goes in straight line rather than generating multiple branches just likes in this picture ?

 A: Lightning branches so that it can reduce the resistance of the path it takes to the ground. As you know, when you attach two identical resistors in parallel, the equivalent resistance is lesser than that of the two in series. Branching essentially does that. It creates more and more parallel paths by which the electrons can flow, thereby reducing the total resistance of the journey from sky to ground. 
The mechanism of how it branches is similar to that of rivers branching. The water streams branch when they approach a solid mass so that they can move with lesser obstruction around the mass. 
A: I know this is a little more than you asked for, but lightning is very interesting. 
A lightning event is usually called a flash and lasts about 0.5 seconds. It consists of a near-invisible stepped leader followed by a very bright return stroke backwards along the path of the stepped leader. Following the first stroke, there may be additional strokes in the flash, following nearly the same path of the first one. There may be slight deviations due to other dim leaders called darts.
Over half of all lightning flashes happen within a cloud and are called IC discharges. The type of flash of most practical importance is the cloud-to-ground (CG) lightning. Other rarer types of lightning are cloud-to-cloud and cloud-to-air flashes. Note that the bright flash that we see is the return stroke, so cloud-to-ground lightning will appear to start near the ground and zoom upward; the initial stepped leader happened first, starting in the cloud.
The typical charge separation in a cumulonimbus cloud results in the top of the cloud having a net positive charge, near the bottom having a net negative charge, and occasionally the extreme bottom edge having a small positive charge. The overall effect is that a large negative charge (of magnitude 15 coulombs) is closer to the ground than the positive charge. This charge structure causes the ground to become positively polarized as negative electrons are ``pushed'' away by the cloud.
Air is normally an insulator, but if the charge separation per distance is too large (either because of large charge or small distance), it can become a conductor. The initiation event is unknown, but scientists currently speculate that either atmospheric radioactivity (from Radon-222) or creation of ions from stratospheric reactions with cosmic rays (solar protons, other charged particles, or high energy photons) can trigger a flash. 
In any event, negative charge moves in steps about 50 meters long through a small conductive pathway in the air, pausing from 20 to 50 microseconds at each step, reaching the ground in a few hundredths of a second. The current in this leaders is between 100 and 1000 amperes. After each step, the leader shifts direction as a new conducting path opens.
After the leader reaches the ground, a tremendous burst of energy is released in the return stroke along the path of the leader as the positive ions in the ground and electrons in the leader combine. This recombination produces a current of 20000 to 30000 amperes, and a temperature as high as 30000 K. The temperature of the surface of the Sun is 5800 K. This tremendous release is accompanied by a bright optical flash, an electromagnetic pulse, and the rapid expansion of heated air.  The typical size of the return stroke channel is 1-2 inches in diameter.
If the initial stroke doesn't resolve the charge separation, more charge from the cloud will travel to the ground along the original path. This is called a dart leader. When it reaches the ground (about 10 times faster than the stepped leader), an additional return stroke occurs along the path. The time between strokes is on the order of hundredths of seconds but could be as long as a tenth of a second.  Anyone watching a lightning storm has seen multiple strokes along the same pathway.
A: This is a slightly delayed answer, but since the theory of lightning is still under development, there should be no harm in speculating about it a bit more.
As was mentioned in other answers, a lighting between a cloud and the ground typically starts at a cloud as a negative current heading downwards. 
Let's look at it in more detail.
First, we need to understand where these negative charges are coming from? 
Those charges are not likely to be free electrons and are not likely to be water molecules: they are negative ions of air molecules. There are always a few of negative and positive ions floating around, but when they are sped up by the field created by a charged cloud, they split neutral air molecules and can initiate a chain reaction that ionizes the air along the field lines.
While the negative ions are rushing toward the ground, making more ion pairs along the way, the positive ions rush back toward the cloud doing pretty much the same thing. But while the negative ions have a long way to go, the positive ions start returning to the cloud and neutralizing the charge in the cloud as soon as the process begins.
As the cloud gets neutralized around the origin of the lighting, new charges from other parts of the cloud are rushing in to keep things going. But they are not going to arrive instantaneously, so we could speculate that the field pushing the leader out will weaken a bit following the initial burst. Perhaps this could cause the leader to pause and wait until the charge at its origin in the cloud is replenished. I am not claiming that this is the explanation of the bursty nature of the leader, but just making an observation. 
Since, as we are suggesting, the current of the leader is formed by air molecules moving upward and downward and making a lot of collisions, we could expect that this fast moving air (wind) will create a lot of turbulence around its path.
With that in mind, it is only natural that some of the ions could stray from the straight path and, if they are energetic enough, could possibly initiate alternative paths. 
So, even if the air was perfectly uniform before the lighting got started, it will be thoroughly disturbed as the lightning progresses, creating multiple opportunities for branching.
I don't know if branching tends to occur at the end of the "steps", but if it did, it would not be illogical, since the arrival of a new wave of energy could require multiple outlets.
A: It doesn't branch. It merges, much like water flowing downhill. Because the charge moves from a large spread out space (the earth) to a small concentrated space (the cloud), much as a stream merges into a river flowing into a larger river meets ever greater capacity for erosion and less resistance. The branching is a sign that the charge is mostly moving UP, not down.
A: Because air in some way like a monument wall for electricity , imagine if you have an air bubble in closed volume in solid wall and the tension of pressure in this bubble is increasing then when the tension will be big enough the pressure is crack the wall, so crack is appear and to understand deeper we need to understand that this wall have structure like solids, so it cracks in specific path according to inner structure, in this way air or vacuum is a solid wall for electricity, also this process in some way chaotic , physics is certain science but there is a chaotic processes which unpredictable, because of to many parameters or parameters that outside of the system.
