Do you think Snape was good at potions because Lily was good at potions?

That's a pretty good point, could be. But I think his study of the dark arts helped a lot.

Definitely he must have been. He tried every possible way to impress Lily. Bad thing is he was interested in dark arts too.

It's amazing, wasn't it? His love for Lily. We only come to know his real insight at the end of the last book. It was all about Love.

I personally think that was just his niche. For all we know maybe Lily was good at potions because snape helped her out. Gryffindor probably still had potions with Slytherin so maybe he gave her a hand and helped her to understand the concept. Or, maybe it was her niche too and it was something they both had in common. I actually think that she was another Hermione and was just a talented witch from a muggle family.