Headcanon because I saw people posting theirs.
Remember that it's probably not canon, but I'd like to think that Steve wanted to propose to Betty at some point.
After the big affair and the fallout of the friend group (mostly his own fault, let's be fr), I feel like he'd find Betty as the most stable thing in his life. And clearly, he has some undiagnosed issues (PTSD, probably), and losing her before he could have the chance to become her husband must've been the final straw for him.
He spent most of his money on hidden labs (and paying our lord and saviorrr Igorrr) because he just wanted to recreate some of the stability he felt with her, which drove him into debts, which drove him into associating with BMP... which led to the plot of the game.
I can imagine him placing the ring he had bought on her finger during the funeral, because he didn't get the chance to propose to her properly. He had planned out the entire proposal (even asking Vivian for advice) and then had to hear about her death.
Welp. Goodnight, people.