So I've had 15 shots at this point. One in my backside, 3 in my right thigh, and 11 in my left thigh. I wasn't informed until recently (2 months ago?) that I am supposed to alternate legs. I'm curious, why is that? To be honest, I absolutely hate doing it in the right, and it never goes well. It either hurts like hell, and/or leaks out. I'd honestly rather not if I don't /have/ to.

I've always injected in the same thigh. My doctors never mentioned switching legs.

I do the same leg every week, too. As does my boyfriend. We've got different doctors and neither have said anything to suggest it's naughty. We've just been told not to stab the exact same place every week, which kind of goes without saying.

I've been told it is important to alternate sites because of scar tissue buildup. When you inject your T, you're not just poking a hole. What's happening is that you're inducing a "sterile abscess" (think of a marble) that actually displaces the tissues until the T is broken down and absorbed (it does not just get absorbed and spread throughout the tissues immediately), which happens over the course of its entire half-life. Over time, this can have a negative effect on your muscle tissues.

But I suppose it's down to personal preference to a point. I'd rather allow my quads and/or glutes the two weeks to rest and repair themselves rather than to keep bombarding one of them with it over and over again.

It's kind of like the rule of resting after weight lifting... its not a suggestion, it's a necessity and for the same reasons.

Or OK... just about everyone has had an IM in their deltoid muscle, like the tetanus vaccination right? Well, for me I get a big ole knot in it for a good three days after. That's why it happens and it's the same thing that we're doing with our T but significantly more often. It's just not good for the muscle tissues to target the same one all of the time. Spread it out and they'll be able to get some time in there to recover from your dose.

It's to slow scar tissue build up.  It's going to happen either way, but by alternating injection sites you can prevent it from happening so soon.