Fat at 25 is a world away from fat at 40. Fat at 25 is largely an aesthetic issue. If you like being big, if you are attracted to big people and confident that people will be attracted to you, go for it. It can be hard to find clothes that fit and you likely won't win any foot races, but you can certainly be a big kid and keep up physically.
By 40, your body's starting to chug anyway. That time your knee got kicked out in a mosh pit isn't just receding comfortably into the past, if you know what I mean. Your back aches because your mattress sucks. You get a weird elbow pain and hope it's not impending elbow cancer. So maybe having a bunch of extra weight on you starts to be more of an issue. You're panting goes up stairs. You don't want to take a trip across country because plane seats are a pain in the ass. You worry about your blood pressure, your heart, all the inside bits.
So you start to weigh calories in verses calories out. You cut back on your beer some. And then you remember that if you could do all that shit regularly, you wouldn't be a big fat guy.
So, trying to lose weight. Not sure if I want to talk about it or not. Definitely don't want advice - thanks, though, heard it. Just putting that out there.