I am learning from the commentary for Classic Doctor Who that some of the original cast and crew have reservations about the new series. They feel that the dynamic between the Doctor and the companions has changed. They don't understand why the Doctor would be lonely - he could go anywhere and get a companion. They, also, felt that he should be somewhat aloof from his companions and be above romantic feelings. (Commentary - "Doctor Who and the Silurians")
1. We aren't making TV the same way we did decades ago. Audience have different expectations and appetites. So. yeah. things change. Nothing wrong with that. I don't mind that he's more involved in his companions. They are also more fully written than in the classic series. They aren't just exposition machines anymore.
2. Of course, the Doctor would feel lonely. The Fourth Doctor goes on how unique he is, he's much older than the people around him, he walks through time and space. He can't grow old with any of his companions, they'll turn to dust before he'll ever die. That sounds like a lonely horrible experience, all of your relationships would be temporary.
So, while I respect the original actors, I disagree with them.