Yah, it could've sensed Parker's powers, as Arcturus mentioned, but I think the symbiote, like pretty much every alien in any movie, is selfish; so in that way, perhaps it wanted to corrupt the happy Peter Parker and get him to hate everyone and have everyone hate him. Look at the negative effect; he was a jerk and didn't care about anyone, even trying to kill Sandman and Harry.
Again, Raimi should've explained this...but you can just add this to all the other things.
What other selfish aliens are there?