[The Doctor knows this. In fact, Scratchman was what he would loosely say was a demon from another dimension, that did try to break into The Doctor's dimension.]

Oi! I wouldn't say it's lonely. I've traveled with a lot of friends.

[But none of them were forever. They died or went home. In a way, is lonely, like Reinette said being a 'lonely Angel' and right now, he does miss Rose and Jack. So, the loneliness is quite clear.]

Yes, I've fought monsters in the universe. But time doesn't run out. Especially, given, that time isn't a strict progression from cause to effect, but actually, from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff.

[A short pause]

Have you seen Back to The Future?