"Vincent and The Doctor", season 5 episode 10, Doctor Who

Amy Pond: We didn't make a difference at all.
The Doctor: I wouldn't say that. The way I see it, every life is a pile of good things and bad things. Hey.
[hugs Amy]
The Doctor: The good things don't always soften the bad things, but vice-versa, the bad things don't necessarily spoil the good things and make them unimportant.