Loss hurts - we find ways to make ourselves hurt less.

With maturity, you'll realize that all people are good and bad, and that it is okay to love people that don't love us back the way we love them. It's not okay to act on it, but it's life... there will always unfair investments (of time, love, effort, etc.) that we'll encounter in lives.

Coping mechanisms - even unhealthy ones - are just that, they are tools to help us cope with pain.

Don't abuse hate too much, if you can help it. Hate is a poison that kills without discrepancy or mercy... it will, as you say, kill your interest, but it also taints the memories, experiences, lessons learned, and relationships with others associated with that person.