How we cite our quotes:
ASTROV: [...] I don't want anything, I don't need anything, I don't love anyone… But I do love you. [Kisses her on the head.] As a child I had a nyanya like you. (1.33-35)
When the Doctor says that he doesn't love anyone, but makes an exception for Marina, we see it as a way of separating out different kinds of love. There's the first kind, which he doesn't feel for anyone, and goes along with wanting and needing someone else. The second kind is a nostalgic love, which is more like the relationship between a mother or nanny and child.
VOYNITSKY: But how lovely she is! How lovely! In all my life I've never seen such a beautiful woman. (1.96-97)
Beauty and loveliness are synonyms, and the fact that Vanya is putting the adjectives "lovely" and "beautiful" together shows how love, for him, goes hand in hand with pretty things. He doesn't seem to respect Yelena for her brains, musical talents, or personality; for him, her outer appearance is all it takes for him to love her. Which makes us think that Vanya's emotions might be a little bit superficial.
VOYNITSKY: [...] No Don Juan has had such complete success! His first wife, my sister, a lovely meek creature, pure as that blue sky, noble, generous, with more admirers than he had students, loved him with the kind of love only pure angels have for those as pure and beautiful as themselves. (1.141-45)
Don Juan is the original Latin lover, a real womanizer, so when Vanya compares Serebryakov to him, it all seems a little bit silly. Serebryakov is old, grouchy, and sick—not exactly what the ladies go for. And that irony, the difference between Don Juan and Serebryakov, is exactly what drives Vanya nuts: the old guy is just as lucky as the world's greatest lover, even though he doesn't seem to have any redeeming qualities.