At the risk of stating the obvious, the main thing I like about this game is that every match is different and presents a new challenge. Every day I play with a new set of players, sometimes familiar, sometimes not.
If I play Outpost as the lone sniper the gameplay is different than when there are two or more snipers. You go from being the hunter to the hunted. Throw in a few folks with proxies and sniper hunting skills and things change further. Sometimes the gameplay of others forces you to change loadouts in order to adapt.
I actually don't mind campers because they can add a fun challenge to the game. When I snipe in the Club I gain an advantage while challenging other players to take me out - which is not that hard. It's a calculated trade-off that I make.
Switching to an atypical loadout and strategy can be surprisingly effective. In Jungle I usually snipe or run and gun. Decided to pull out the shotgun yesterday and stay clear of the open areas and finished in 2nd place.
Anyway, just some random thoughts about why I and possibly many of us keep coming back for more.