The chapter where my MC is going to propose to his girlfriend is coming up. I'm thinking about having her not answer him right away after he pulls the ring out and asks her, but I worry readers would question if she actually loves him or not. She does love him, but she's only 18 and just now starting to get an idea of what she wants to do with her life. So, would not having her say yes instantly make you question if she loves him?
If the narrative up until that point made it clear that she loves him I wouldn't suddenly question it because she didn't say yes immediately to a marriage proposal. You can love someone without wanting or being ready to marry them - there's about a million possible reasons why.
I think it would depend on the tone of your novel too. If it follows the more generic, cliche sort of story, then yes, one might question why her response wasn't instant. But if it's a more down-to-earth, realistic portrayal of love, then no, not necessarily. I think it depends on how you've built up the reader's expectation - is this an ideal world or is this world one that reflects reality fairly well? Not sure if I'm making myself clear... Also, making the reader question if the girl really loves the guy might not be a bad idea it would add to the tension, like OH NO!