I'm someone that played a lots of sports when I was younger. I remember playing almost every sport possible in P.E class in school. There were some sports that my mom had me try like basketball because she thought I would grow taller. I gave it up after a year because I'm short and I just sucked at the game so the team actually didn't give me a lot of game time which I was fine with as it was bit stressful knowing I was letting the team down. My mom also tried to get me into golf but I didn't like that game either. The games were long and you really needed a lot of patience to play. I remember trying put the ball in the hole and it was kind of annoying when it kept missing. I also remember trying cross country for a year in highschool but I let that one go as I didn't have time.
In the end, I ended up focusing on two sports, tennis and soccer (football in other countries). I'm using to calling it soccer so it's hard to change. For me, I was really passionate about these sports at the time. I played competitively in both which also got me to watch these sports. With all this experience in sports, I feel I can understand where a person gets the passion from watching sports.
Personally, watching these sports kind of gave me an idol to look up to so for tennis my idol was Roger Federer and for soccer I liked Messi because he was super skillful and had good ball control. My first point is that some people become passionate about watching sports because they play the sport themselves and feel a personal connection to the players or teams.
Of course, that's not the only reason for people's passion in watching sports. I think it could also be part of the country. For example, if I'm not wrong countries like Brazil and Spain are big soccer countries. It's rooted into their culture so the passion of watching a sport could come from the cultural experience. Then there's family which could be another reason. It's possible that parents watch the sports and as the kids grow up, they also tend to watch the same sport and support the same team.
This is all I could think of at this moment. If I missed any reasons and you're passionate about watching sports do let me know in the comments below. Honestly, I don't have the time to watch the full game of sports anymore. I usually just watch clips that highlight the goals in soccer or highlight the best points in tennis.