I've been told that too. But, I think people telling others how to feel is wrong. How you feel is how you feel. It's not like you asked them how to deal with it. They just impose how they think you should act and think for their own purposes.

That being said, dwelling (if done too much) can be bad because even though the present is part of the result of the past, it isn't the past.

The past does effect you though, so anyone that doesn't understand that and lacks in empathy can get lost.