Doc is someone who cares about helping people while not being that much of a people person himself. Once he does his heroic deed, he dryly comments on how ordinary and unremarkable it was (“Just passing through” “Eh, lucky break”), and attempts to extricate himself from attention as quickly as possible. While not rude, if someone engages him in conversation, his replies are simple and short. A lot of his actions seem to send the message, “I’m a boring Good Samaritan, please go away.” But since he often does this in such an effortlessly smooth and cool manner that it has the opposite effect.

He doesn’t mind people, but he prefers to be to his own company.
Still, Doc seemed charmed by Littlefoot and does his best to help the boy. He gave advice on how to deal with Littlefoot’s bad luck problem drawing from his own experience. I think part of the reason he pushed Littlefoot to see his grandfather as a hero was to value what he had and make the boy happy…and to get attention off him. Doc can only take so much cute hero worship.
Nevertheless, Doc appeared to remember Littlefoot. Being a kid who get caught up in dangerous adventures with his friends does lend to making him more memorable. Since Littlefoot has a tendency to help people like he does and he has no kin of his own, it isn’t too much of a far shot that Doc would return to introduce his mate Dara to Littlefoot and his grandparents. I think his fondness for Littlefoot only strengthened when the boy called him out for judging the gang’s friendship with Chomper. He doesn’t much like the hero worship he can sometimes get and he appreciates that someone can straight talk to him when he’s being wrong about something.
As for why Doc suddenly has a mate – well, even introverts like some romance. From what we hear, Dara doesn’t mind traveling with few companions either even if she seems the more sociable one of the two, so that must make having a relationship easier for Doc. For all we know, she might have the same restless spirit as he does in exploring the world.
And speaking of exploring the world, Doc seems to have seen enough of it to know some locations aren’t as unique as some might think. Doc mentions “This [Great] Valley’s nice, but there are others just as nice out there somewhere.” Knowing there are places equal to or greater than the valley, he can’t stay in one place for long and is always on a journey to find some place better. If he stays in one place too long, the wanderlust he built up all of his life starts to itch at him. He admitted this is somewhat out of habit but I think he does like exploring, seeing new places, and maybe a few times meeting new people. Though I also think he wanders because “Crud, being around these same people for too many days is making me stuffy, got to get away, byyyye!” (Now I have the amusing mental image of a storm pinning Doc in a community for days and weeks on end, which makes him all but crawl the walls by the time it ends and barreling out of the community before anyone can bid him farewell).
A revealing comment Doc made while giving Littlefoot advice on the bad luck issue is “Change what you can, accept what you can’t.” I’m under the impression Doc does his best to help and change others in unobstructive ways. If others are too stubborn about not changing, though, he isn’t going to attempt moving what he believes is a mountain and moves on. He’s not one to really cause waves unless it’s of supreme importance and he doesn’t feel it’s his place to change who people are. Experience has probably taught him being nosy and trying to change others only makes things worse, so he makes do with what he can accomplish. He seems to have a realistic, pragmatic view of how the world works. It isn’t negative but you have to pick and choose your battles, for not all of them are winnable or worth fighting.
I wonder how much of that might be influenced by that experience and how much of it by his solitary, wandering lifestyle. If he was a bit more social and established deep roots in a community, would he retain the same attitude or work harder to solve the seemingly insurmountable problems since he has a personal stake? I can picture Doc and Littlefoot musing this as they compare notes with how they deal with problems. It’d be interesting to see how Doc might react to such a conversation.