Is there a reason for levers to be reversed on bikes with hydraulic brakes? I recently bought a MTB with hydraulic disk brakes, and am finding really hard to adjust to the braking system. From the very beginning I was cycling with the front lever on the right. But now it’s reversed, and it’s really confusing me a lot.

I asked my mechanic about this and he says front brakes on the left is the European style of braking. So I should get used to it.

But, why exactly are brakes reversed on such bikes? And to add up, my mechanic claims that the brakes cannot be switched. Is this true or, am I just being punked?
It's not for hand signals. "Left hand only" signals were originally designed for cars. You couldn't use your right hand out the window, so you could only use your left. Using both hands to properly signal on your bike is far more clear. It's used on motorcycles due to the fact that the throttle is on the right hand, so you want to keep that engaged at all times.

Reversed brake routing (front on the right, back on the left) is called "moto" routing and is primarily used in MTB and cyclocross bikes. On a CX bike, it's done so you can have greater controlled braking with the left hand on the back brake while your dismounting your bike for a barrier and your right hand is on the top tube. The front brake offers more power, while the back offers easier minor speed control while approaching a barrier.

To answer your initial question, there is absolutely no reason the brakes cannot be switched on your bike. Either the mechanic is lazy or doesn't know how to work on hydraulic brakes. In either case, you should take it to another more capable shop.