There's actually a few games I return to really frequently, and here's my reasons for all:
1). Until recently, I've always returned to WoW, because I like the setting--it's one I've literally known since I was a wee babe playing WC2. Much like Ladysteel said, this cannot be forced, but maybe to help those interested get lost into this universe, it may be a good idea to make the lore easily accessible, which should also help the RP community to prosper, as well. Maybe some points being vaguer than others, but most of it should be there, nevertheless.
2). Another MMO I usually return to whenever possible, as well as where I draw my username from, is SWTOR. The reason I've returned to SWTOR, and even sunk a fair bit of money into it when my circumstances have allowed me to, is my guild. I found an amazeballs, small community of dedicated RPers who were almost as tight-knit as a second family. Unfortunately, we have since disbanded. I hope to try my luck to run into atleast a couple when this launches, if my half-eaten potato can run it.
3). I used to return to DDO every once in awhile, because of the sheer number of possibilities for how I could play my character. However, Daybreak has since "partnered" with the now-independent developers, aaand I make a point of avoiding them at every turn I can, as they are the bane of MMOs and MMO players everywhere.
4). Lastly, there's Champions Online and STO, which I still find myself returning to often enough. I like the sheer levels of customization in both. The former makes it relatively easy for me to recreate my characters, "dead" or otherwise, while the latter makes me truly feel like it's my character's story unfolding. Both also have me happy with their biography function, but both mildly anger me with their lack of working /roll and /e functions, which effectively screws the RP crowd.