Life is too short to spend your precious time trying to convince a person who wants to live in gloom and doom otherwise. Give lifting that person your best shot, but don't hang around long enough for his/her bad attitude to pull you down. Instead, surround yourself with optimistic people.
Zig ZiglarIf you go out looking for friends, they'll be scarce. If you go out looking to be one, you'll have all the friends you need.
Zig Ziglar