Advice from a male perspective

I've been with my boyfriend for about 3 1/2 years. We dated for 1 then we've lived together for 2 1/2 years. We have a very good relationship. He's my best friend. We've talked about marriage before & neither one of us really wanted to take that leap. We agreed that we both were just happy to live together & have a relationship we someone that was a lover but also we're best friends. He recently got in trouble with the law (DUI) & may have to do some time. It's been very stressful for both of us...the fear of separation anxiety & financial worries. Today out of the blue he said to me "You know I'm going to make you my wife." I said "No I didn't know you were thinking about that, where did that come from?" He said, "I've never loved a woman like I love you & I want you to be my wife." I was completely shocked. I told him I hadn't even been thinking about marriage.
I just don't know where that came from. I need advice from a man, why is this coming up? Could he be afraid he'll lose me if he has to do jail time? I've never pressured him about marriage or even mentioned that I wanted to get married. What could he be thinking? Is it just fear of him losing me? Any advice??

People change there minds as they get older generally and even someone so dead against marriage may change there mind at a later date. It could be the fear of losing you should he go away (that seems harsh if its his first offence but hey im from England but as a policeman we do things differently here) or it could be he has decided that he does want marriage.

Thank you for your response. No, it's not his first DUI offense. So I believe he will get some time, maybe 6 months. But we are so use to being together all the time. When we're not at work, we're together. We watch the same shows & movies & like to do the same things. I don't know I was just so shocked & I've been thinking about it all day. I just wanted an outside perspective. I'm going to ask him what made him say that & why he's thinking about it. Thank you.