People that say thank you or your welcome gives me the impression that they're kind and respectful and it makes me feel better. Much better than not saying anything at all and possibly leaving me with the false impression that they probably don't care. Every time I help somebody and get no response, it always leaves me wondering why they didn't acknowledge it. Some may still appreciate what I do for them, but It's always nice to get some recognition every now and then.
When I know somebody that truly appreciates what I've done for them, It gives me the confidence and motivation to continue helping others as much as I can.