Why won't he propose?

Me and my boyfriend have been together for 4 years. We practically act like a married couple. We live together, and do everything together. But every time I bring up marriage, it starts an argument because he never wants to talk about it! I've tried not talking about it, and I didn't for months!! But after so many, I brought it up again, and he flipped! He is deploying in 2 years, and a year ago I thought we both agreed to be engaged for two years, and married a year before he leaves. And now we are down to 2 years, and I just don't want to be engaged for a couple months and rush into a marriage! I feel like my way is so much smarter! Can anyone give me advice? How do I talk to him about this without him flipping out? And how do I get him to understand that being engaged now and for a longer period of time is smarter? I just need advice for everything on the topic of engagement😩😩😭