Why do magnetic field lines behave the way they do?
Because they map out "the state of space". See Einstein's 1929 history of field theory where he said this: "The two types of field are causally linked in this theory, but still not fused to an identity. It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds". A field is a state of space. The state of space around your magnet is not the same as the state of space around your apple.
So I learned that the bar magnet makes a magnetic field and the field lines have a certain shape which kinda resembles 2 semi-ovals.
Yes. The magnetic field of a bar magnet is like that of a solenoid. See this depiction from Rod Nave's hyperphysics:
If you reduce the number of coils in the solenoid, it's like shortening your bar magnet. Keep doing this and eventually you get to a current loop, where the magnetic field tends to be drawn like this:
And if we pass current through a conductor then it makes a magnetic fields and the field lines are in the form of concentric circles around the conductor.
Yes. You can draw it like this:
The magnetic field lines map out the state of space, which determines how an electron will move. If you throw it from right to left it into the current, it will move in a clockwise near-circular fashion. Note though that a positron will move anticlockwise. This is because "it takes two to tango". The motion of the particle depends on the space its in, and on the particle.
My question is why? Why do magnetic field lines take the shape that they take? There must be a reason behind it.
There is. It's because the magnetic field around the straight wire has a rotational symmetry, and it gets weaker with distance. So we draw concentric circles, with bigger and bigger gaps between them. The charged particle moves round a larger-diameter near-circular path when it's further from the wire.
Why aren't the field lines of a normal bar magnet shaped like the field lines of a conductor carrying electricity? And vice-versa?
Because they're like the field lines of the current loop instead. To go from the straight wire to the loop, you just bend it round. Remember the depictions aren't perfect. The magnetic field around the wire has a cylindrical disposition, the field around the current loop has a toroidal disposition. To go from one to the other you just bend the wire. Maybe what would help is if I drew two sets of concentric circles. Bend this into a loop:
and you should be able to see why the magnetic field looks like it does in the second picture above.
Please explain it to me..i know that it delves very deep into the fundamentals of the working of science but still...please tell me the reason if anyone knows it.
I know all about magnetism Marty. It's all pretty simple when you understand it. Unfortunately, some people seem to prefer mystery. Sorry about that.