Realism
Chekhov believed that theater should reflect life. "What happens onstage should be just as complicated and just as simple as things are in real life. People are sitting at a table having dinner, that's all, but at the same time their happiness is being created, or their lives are being torn apart," he wrote.
In his efforts to craft plays this way, Chekhov meticulously recorded bits of conversation and quirks of personality, just as Trigorin does here. It's a good bet that Chekhov based Nina's flattery on his experiences with young fangirls, like this gem:
Nina: Oh, but I'd think once you've known the thrill of creation, all other pleasures must pale by comparison. (1.139)
And the moment when Nina gives Trigorin the medallion came right out of Chekhov's own life. A smitten young lady once gave him a medallion referencing a line from one of his stories—the same line he quotes from Trigorin's fictional Days and Nights.
Chekhov eventually loaned this medallion to the prop master for use in the production of The Seagull, and even gave it as a memento to congratulate the actress playing Nina. Romantically sensitive? Not so much. Creative use of reality? Totally. And there's lots more good gossip about autobiographical sources where that came from.