Source games were my first introduction to the "adult" world of gaming almost 15 years ago. First time I played Garry's Mod was in a conputer bar/LAN gaming center (the place where you rent a workstation to play games on it). Then I got Steam, CS:S, and GMod. Source & GM have influenced so much of my gaming existence that I've got to flatly say Source games are the best, and have a special place in my heart, GMod especially.
GMod is the pinnacle Source game. That's why I still play it. Too good to put down and if you're bored there's always some niche server with crazy mechanics that you can play.
But as for why I keep coming back to Aether? It's just a nice community. Funny toys w crates and a good crowd.