why same charge repel and opposite charge attract each other?

Every object has a tendency to lose energy (lower it's energy).

Suppose you have two charges q₁ and q₂ ( same/opposite ) . As the energy of a system is given by : 

P.E = k * q₁ * q₂ / r

k = constant for a medium .

r = distance between charges.

As the body wants to lose energy so P.E (Potential Energy) has to be minimum. For which :

r (distance between charges) should be maximum . Therefore the same charges move away to increase the distance between them (r) .

Now , as if q₁ and q₂ are two opposite charges. so their product will be negative :

P.E = - k (q₁)(q₂) / r

Thus for P.E to be minimum : r should be minimum.

Thus opposite charges move towards each other to decrease the distance between them (r) .