Why did Eric van der woodsen from Gossip Girl try to kill himself?

Because he was lonely. Lily said to Rufus in Dare Devil (episode 5 season 1) "he's not a bad kid, he's just a lonely one."

Erik also said to Jenny in Poison Ivy (Episode 3 season 1) '...so, we [Erik & Lily] moved into the hotel, and it was just the two of us. That was really lonely..."