Why would we need to ground an AC source I'm new to this field hence this weird question. Why would we need to ground an AC source? Why wouldn't it be enough to have just one pole to get an AC current going? I understand why it wouldn't work in DC case where current is flowing in one direction. However, in case of AC source where the current is not flowing anywhere but rather just oscillating back and forth it's not that clear to me why connecting load to only one pole wouldn't work? Thanks.
 A: There are two related issues here, and I'm not sure which is your actual question.
In household AC circuits with three-wire cables, the ground is primarily a safety system. Somewhere near where the utility power reaches your electric meter, your ground wire is actually connected to ... the ground, via a stake in the dirt or a connection to your plumbing. The earth can source or sink a great deal of charge without harm, and so a short between a live wire and a grounded case is much less dangerous than a short between a live wire and a loved one. If you look closely at wire runs through metal conduit, (usually outdoors at home, more common in industrial settings) you should find that both ends of the conduit are connected to the ground wire; this shields the power cable somewhat against acting like a radio antenna.
You can absolutely operate an AC circuit without a ground wire; the was generally how systems were installed in the several decades before grounded systems became standard, and you still find them sometimes in old houses.
I think it's more likely that you're asking about whether you can operate an AC circuit without a return path for the current, since the current is just sloshing around. That's possible too, though the details for the circuit are a little different. Here's a photo of one:

As you probably remember from your experience with these "one-wire" systems, you don't even have to be in contact with them for the field to work! The electrical energy is carried not by the charges, but by the fields created by the moving charges, and the energy flow goes in the direction determined by the Poynting vector. This is even the case for a DC circuit, which has this sort of field between its "source" and "return" lines.

What would a one-wire, contact transmission system be like? The voltages in the one wire would have to be regulated relative to ground (not the ground wire, since we've eliminated that, but relative to a stake in the dirt somewhere). Essentially you'd be using the earth as a "return line." Since the return flow would presumably spread out over a large volume under the earth, you'd effectively have a transmission line with a very large gap between the source and return currents, which would give a tiny capacitance and a huge impedance. It could be made to work, but it would be much more finicky and unpredictable than the system we have now.
A: You need a return wire to complete the circuit.  Otherwise the electrons would all try to pile up at the end and the current doesn't flow.  In $60$ Hz circuits, the current flows in one direction for $1/60$ second.  That doesn't mean you need a ground wire-AC works fine isolated from ground.
A: If you have only one wire, leaving its end open, you still have current flowing in the conductor, but the efficiency will not be optimal due to impedance mismatch. Since the aim is to transport power, we need the other wire to optimize the efficiency. 
