Interestingly enough, pebblesflinstone, I heard a very good answer to this question in a movie. It was a 1995 spy thriller called Bad Company starring Laurence (not Larry, I understand) Fishburne and Ellen Barkin.
This was an extremely underrated film about CIA agent Nelson Crowe (Fishburne) who goes undercover in a industrial espionage and sabotage firm for private enterprise called The Toolshed, headed up by former CIA man Vic Grimes (Frank Langella). His "right-hand man" was another former agent called Margeret Welles (Ellen Barkin).
The idea is that "The Agency" wants to get rid of Vic Grimes, nail him on tax evasion, and take over The Toolshed for their own purposes. This is crucial because the Central Intelligence Agency wants to have its own privately financed, off-the-books, intelligence gathering and black operations boutique - at no cost to the good old American tax payer.
Now, before I actually address your question, let me say - as I usually feel compelled to - that the chemistry between Barkin and Fishburne was electric. I found myself wishing those two could run away together, leave all the nastiness behind, and set up as slightly shady private investigators in Antigua, or someplace. But it was not to be. Everybody, included this sizzling couple, ended up dead.
At any rate, to continue, there came a scene in which your very question was asked and answered - I think convincingly.
Yada, yada, yada, and there came a point when Ms. Welles proposed a consipiracy to Crowe, who by that time had become her lover. "How about half the Toolshed?" she said to Crowe, as they were lying about naked after having made love (and I think made love is actually the appropriate description here) an drinking scotch.
"Who'd own the other half?" Crowe asked.
"Someone willing to share the rest?" she said.
When she doesn't answer, Crowe laughs and says "Grimes goes."
That's the plan. Welles and Crowe will kill Grimes and take over the Toolshed. Crowe goes "the distance" with her, as he has been ordered by his CIA handlers to do.
On the night before they do it, as they are finalizing preparations. Margeret looks up and says, "You know why I hate Grimes?"
Nelson Crowe says, "Probably because you owe him so much. If its a choice between hate and being grateful... Who wants to be grateful?"
There your are, some people have a problem being human and vulnerable and grateful.