2
1
4
3
Why:
2: Vanity, obviously, but also to monitor my progress. I find I don't see myself accurately in the mirror, so keeping track of bf% helps me keep going in the direction I want to go and helps tell me if the weight I'm gaining (or losing, depending...) is muscle or if I'm just getting fat (or losing fat, again depending...)!
1 Because I'm kind of mentally hung up on getting over 200 lbs while staying under 10% - and really, would like to get closer to 8%... go back to #2 above...
4 Because I'm vain, obviously. 3rd on the list because like I said, I don't see myself accurately most of the time. Sometimes I look in the mirror and think, "Damn, I'm getting pretty big, looking pretty good!" The next day, or even later that day, I might look in the mirror and think, "I'm not making any progress at all!" Or that I haven't changed much at all in the lastyear. Meanwhile, I've outgrown my shirts and have dropped a waist size..... At my heaviest (205), I discovered that I was at 16 or 17% bf, which was the most fat I've ever had on me, and didn't even realize it was happening! Just kept eating and lifting, getting stronger for sure, but abs disappearing slowly until nearly gone altogether and still thinking I look good...
4 Because if I'm getting stronger I must be making progress. But my strength goes up and down day to day, so I don't focus on this too much as long as I'm generally getting a little stronger as time goes on. Plus, and maybe this is because of my age (45 - almost 46), if I push it too much I end up getting hurt. So I try to make slow progress in small increments. If I feel strong one day, I'll push myself, but if I'm tired or for whatever reason not feeling as strong, I'll go as heavy as I feel comfortable with, concentrate on good form and pay attention to how I feel.
4 and 3 could really be right on the same level.