For me it's my love for supporting my party, doing what I can to support the party synergy and help everyone play more effectively. In dungeons, I love being the person who can make the runs go so much smoother, using my buffs on party members and debuffs on monsters, keeping everyone safe with heals while also making a big contribution to group DPS, being able to say "let me handle these mechanics/adds so the rest of you can focus on DPS"... It's simply that it makes healer such a useful and powerful addition to the team, playing the job fully makes the run so much smoother, not to mention the healer DPS lessens the amount of damage taken by everyone and increases everyone else's DPS capacity (since the fights are shorter).

And for endgame content, I guess I love breaking my own records, trying to optimise my play more and more in a certain fight, and aiming for a higher ranking at FFlogs.