Yeah, I keep seeing people saying "But if the Doctor's so lonely, why does he push people away? He's got all these awesome people surrounding him!" But... it doesn't work like that. You can't just magically open up again after you've been hurt; it takes a lot of time to build up trust and regain the ability to let other people in. And it's particularly difficult for the Doctor because he's an alien, so there's an extra distance between him and everyone around him. (Which is part of why he gets so fixated on the Master--finally someone who can understand that "alien" side of him that the humans will just never get.)
Yes.

Okay yeah, the Doctor longing to connect with people and then taking forever to acknowledge Martha seems stupid when you word it like that. But he's lost his people and opened up to Rose about that and then he lost her. So, no, he's not going to connect easily with someone else again. People don't work like that. And people of all sorts in Whoverse - aliens or not - tend to work the same in that matter.

And him losing the Master and all those people in VotD? No wonder they're bringing back the Doctor. He's in a place where he needs to connect again with someone he's known in the past. Like he did with the Master and Jack.

And bringing up like he was before? When he refused to say a proper goodbye to many of his companions and never goes to look for them again even though he still clearly loves them dearly? This is not something a healthy person with a time machine does. School Reunion explains it all.