Whenever someone was bullying someone else Eren would go right up to the bully and punch them in the face. 

This caused him to get hurt a lot and his Dad would have to patch him up.

When he became friends with Armin he started getting into fights more often since Armin was always getting bullied.

But when Mikasa came into the picture Eren didn't get hurt as much and Armin didn't get bullied as much.

Eren hates it when he sees the weak being degraded. This is because he knows that even though they might not be strong they still have some talent in them or something about them that makes them stand out.

This also contributes to the fact he hates Titans. It's because the Titans treat them like they're nothing. Just like the bullies.