Is this normal? Am I overreacting?

My boyfriend and I have been serious for about 3 years now. He only has 1 (male) friend. They are extremely close, they talk on a daily basis and see each other multiple times a week. They live minutes apart. They are also business partners, so you can see how this person plays a huge role in my boyfriend's life.

I have seen him dozens of times, no more than hello. We have never actually spent any time together. And it has started bothering me. I know I might be overreacting, because my boyfriend is quite private about his life, but WE HAVE NEVER BEEN OUT TOGETHER, or hung out at home. Not once. I get it that he likes to spend time without me, but to never include me seems strange.

It started seeming strange when, there were the accidents, in which me and his friend met in front of my boyfriends house and had small talk (like hey, how are you, how was your day). And my boyfriend mildly freaks out and starts asking me what we were talking about. It didn't bother me as much, but now I'm starting to get suspicious. His friend plays a huge role in his life, so why have we never actually spent time together?

It's not like he is completely hiding me, his friend has met me and says hello. His friend knows we plan on getting married in the future and is going to be the best man. My boyfriend introduced me to his family within our first month, so I doubt he is hiding me or keeping our relationship not serious. His mom has told me all of his embarasing stories, so I doubt he is "hiding" me from hearing about childhood things.

Whenever I bring the topic up, he gets defensive and starts saying how he likes to keep his life private (it's not like I'm going to talk about relationship drama, but hey).

He is either ashamed of me, they are either hiding something (which I doubt is cheating, but who knows), or he really likes to keep his life private and I might be overreacting.

Please give me advice. Thank you so much for reading.