This question is related to this question. If I write a thesis and encounter a small problem that I can't resolve on my own, maybe because I didn't sleep well this day, and someone, like my mother or anyone else, suggests something like, 'Oh, that's trivial, you could use Theorem XY or fact YZ,' and this helps me overcome the issue—even though I knew these facts but had overlooked them; e.g. like looking at the sky and not noticing a plane flying overhead. And I still prove everything myself in the end completely out of my head, where the main idea is from me. Is it then a big issue to not give that person credit because it might seem awkward in a thesis? Of course this person can guess that I appreciate the hint.
At some point, to put it a bit exaggerated, it's like trying to estimate something, and someone realizes that it can be trivially managed with the Cauchy-Schwarz inequality or something similar. It's not like Perelman using the Ricci flow to solve the Poincaré conjecture.
:-)
(This is becoming recursive...):-)
Another (selfish) reason to be benevolent: One whose assistance has been properly appreciated may be more willing to provide help again when the inevitable next blockage occurs. My previous comment should have been merely "Think about The Golden Rule: Do unto others as you'd have done unto you." or, Pink Floyd: "I mean good manners don't cost nothing do they, eh?"