It depends on the person. The world is made up into two types positive and negative. Some people give up to easily and will dwell in their negativity. Those I just leave alone.
I would reply by explaining to them the basis of the Law of Attraction and pointing out that what they said is a cop out and why. However, I don't spend a lot of time on pessimism. If they insist on standing their ground, and dwelling on negativity, I let them. Then I bless them and let them go.