What motivates you to run old content?

If I were going to be honest I’d admit up front that I haul my alts through older content for a variety of reasons. A solid half of the time it’s for transmog, and then the rest of the time is divided between enjoying feeling overpowered by blowing up old raid bosses, and because my wife wanted to get transmog gear or a pet. Every time they add a new pet to an old raid I know I’m going to end up spending time in there. This isn’t an issue. I love old content, and I especially love farming for stuff in it. I find it relaxing and fun.

But other people need to be motivated to do it, and so I always wonder — what makes you decide to go back? Are you adamantly opposed to it? Will you only go back for rare, hard to get mounts? Do you like to farm up pieces for that legendary mace, sword, axe, or daggers you never got on your main, or seek to get on your alt? I keep meaning to get myself a Val’anyr on my Paladin, but I always forget to.

Thus my question — if you run old content, why? What gets you in the door to take another shot at Ragnaros, or the Lich King, or even Scourgelord Tyrannus?