Well, Snape is a very self-centered and selfish person. He is not perfect - not one character in Rowling's universe is - and so, he thinks he is right in his egotiscal way.

He thinks that because he was mistreated, he has the right to mistread others, especially the ones who mistreated him (actually, that's also childish), and the children of those who mistreated him.

I also think that part of the reason he is mean to Harry is because he really doesn't believe that Harry should be treated as a special boy, he mistreats him to make him more down-to-earth, to MAKE him a better potion-maker/occlumens/etc - he probably was brought up that way by his mother and father, and thinks that's the right way to go.

And to me, his motion is not as obscure - he does everything that are better to HIM, no matter if it's bad or good for everybody else. Again, selfish.

I know you can related that to real life people, maybe it will make you understand better! ;)

And I never thought I would say this, but I need a Snape icon for this discussion! XD