It isn't anymore complicated than this.
I play games for the enjoyment. I could really care less about levels and such as I would rather they should simply come naturally through the gameplay. Personally, I've never been one to "grind" a particular mission over and over again, etc. Even if it's not the most efficient way, I prefer to mix it up by doing different activities and such to gain cash or levels. That's exactly why I only reached around lvl 65 before I took a break.
I began to find the game too monotonous for my taste. Not much more to unlock or buy (at least those items I wanted) and most of all, I' had already done all of the activities many times over. ....except most of the PvP events which I'm just not in to.
I don't mind the grind but when it stops becoming fun, that's when I either call it quits or move on to other games for a while. This has absolutely nothing to do with wanting a way to shortcut the game because that also defeats the purpose IMO. It's the incentives for advancing in the game like this that helps keep me going but again, only so far if the content / gameplay begins to feel stale.
"Grinding" in a game can feel rewarding and enjoyable or quite literally like a grind. .....it's when it begins to feel like the latter that I consider taking a break for a while.