While it doesn't seem like much at the time due to Thomas also giving selfish reasons to help Harry through the first few books, it all takes another meaning when you (and Harry) find out that Thomas is Harry's half-brother and has been subtly looking after him.The Reveal via soulgaze was heartbreaking in the good way.
Hell, Thomas has just retroactively proved Harry's 'familial dementia' theory. And it is beautiful.
The moment Dresden accepted the fact that Thomas was his brother
"What do we do?" he asked. "I'm...I feel lost. I don't know what to do."
"I think I do," I said.
Instead of answering him, I offered him my hand.
He took it, and I drew my brother to his feet.
After Harry and Thomas share the soulgaze where they both get to see their mother, Harry has a brief but powerful moment:
The aching, lonely old hurt was overflowing me. But I suddenly found myself laughing. I had seen my mother with my Sight. I had seen her smile, heard her voice, and it was something I could never lose. Something no one could ever take away from me. It couldn't wholly make up for a lifetime of loneliness and silent grief, but it was more than I ever thought I would have.
In a twisted way, Kincaid mentioning that he actually kind of likes Harry.
Genosa's people covering for the wrongly-accused Harry to the cops by wiping off the murder weapon he was supposed to have killed someone with and not giving the police any leads, because he helped save one of them.
After Harry kicks Ebenezar out for being a Blackstaff he's sitting in his apartment, feeling alone and sorry for himself. Then the puppy he saved comes up and starts licking him, and Mister starts rubbing up against him too, almost as if to comfort him. Then, Mister starts batting at the puppy and Harry laughs. Maybe I'm a sap, but it really made this troper tear up a little bit and get all warm and fuzzy.
I laughed. I couldn't help it. The world might be vicious and treacherous and deadly, but it couldn't kill laughter. Laughter, like love, has power to survive the worst things life has to offer. And to do it with style.
Practically every scene with puppy Mouse qualifies, for that matter.
Thomas using up all but the last dregs of his savings to pay off Kincaid for Harry.
During Harry's sparring session with Murphy, she suggests old Detective Malone's nephew, whom we just saw spar against Murphy, as possible muscle for the raid on the Black Court. Harry refuses, not wanting to cause that particular family anymore pain. The Heartwarming, though, comes from Murphy's explanation that the kid knows what Harry did for his uncle, and would walk through fire for him...
Harry gently prodding Murphy to talk about her issues with her family, as he instantly notices she's on edge and even though he has almost no experience with family, he's more than willing to try to help out.
Harry and Marion Murphy's conversation, made even more special by the fact that this is their first time ever meeting. It's so very sincere the whole time. Harry goes out of his way to remain respectful towards her while he disagrees with how she sees what Murphy does.
Marion: Will you take care of my daughter?
Harry: Yes, ma'am. Of course I will.
Harry seeing Murphy in a dress for the first time perfectly straddles "utterly adorable" and "absolutely hilarious." And then there's the scene later on where she's feeling uncertain and he gently compliments how it looks on her, thereby cheering her up.
And YMMV, but Harry's Green-Eyed Epiphany when Kincaid takes off Murphy's pants can be seen as heartwarming since it's basically the first time he consciously realizes he has romantic feelings for Murphy.