It does not do to dwell on dreams and forget to live.
The other thing I have found, is that the more I become fixated on the idea, the harder it becomes for me to act on the reality of it. Or in one of the cases in my life I'm referring to, the more I become fixated on the idealised image of a person, the more nervous and detached I become to the idea of actually talking to them in reality.
As well as this, the hurt that comes if the thing does not come to pass, is not worth it. I've experienced this so often; I imagine my entire life hinged on the eventuality of one circumstance, and then when that circumstance doesn't happen, my entire perspective is thrown out of proportion (e.g. the whole ending up at a uni that wasn't my first choice thing).
There's also the fact that reality is never quite as big and spectacular as it appears in our head. We are constantly taught through films and books what an 'ideal' life should look like, but it's very rarely what life looks like. Our expectations shouldn't be shaped by these things, because a) it's unrealistic, and b) it does the beauty and unpredictability of real life a disservice.
There's so much in life that I have to be thankful for (I made a list the other day and there was just so much to say); and it's not fair to myself to keep coming back to ideas rather than the reality in front of me. It's not fair because although my imagination is pretty great, it can never be as great as God's and he's the one who's planning the path in front of me, and he's the one who is painting the picture that I somehow seem to keep ignoring. To think that my version of events is better than that, is thinking I'm better than God, which I'm clearly not.
So Dumbledore is definitely right. It's super hard to not dwell on our dreams and the desires of our hearts, but when it's making us forget the incredible reality of the life we've been placed it, is it really worth it?