I think of skinny and fat as opposites, backed up by word uses like the "Fattypuffs and Thinifers" - https://en.wikipedia.org/wiki/Fattypuffs_and_Thinifers - it's amusing that people go to the gym because they don't want to be fat, and people go to the gym because they don't want to be thin.
Are there other things in life where people go to the same place for, at a glance, opposite reasons?
Technically they’re both at the gym, but they’re probably doing mostly non-overlapping activities in separate spaces. (Which is a shame because building muscle mass is an often overlooked way to lose weight)
There are different things you can do at the gym, but more than that, with many things, the best/healthiest place to be is between two extremes rather than on either end.
Not just with weight, but with nearly everything in life: try to find a balance. Between time for yourself and spending time with others, between socialism and capitalism, almost every -ism, belief or drive taken to extremes becomes harmful, but can be a useful contribution to a balance between different concerns.
Are there other things in life where people go to the same place for, at a glance, opposite reasons?