Firstly, I love that word. Transitions in general life are always embedded in a myriad of emotions from excitement and joy to stress and worry, often all of the above, sometimes lacking any of the positive elements. However, there is usually some tension involved and that brings us to transitions in dressage.
I used to not like riding a lot of transitions although I've always been taught how useful they were. I didn't like riding them because I've always hated how crappy transitions felt: The horse hops up in the front, the poll comes up, the back tenses and it takes 5 strides to influence the quality of the gait after the fact. It takes focus and a good seat to ride good transitions, both of which take time to develop, so if one lacks in either department, it's a happier place to stay away from them. Not so much in the good riding department though.
Now I can't get enough of transitions, because they go exactly where there is usually room for improvement: Throughness, over-the-back-ness, the horse honestly reaching for the contact, the prompt response to the leg initiated by the hind leg pushing off.
So let's think about those two necessary ingredients for the good transitions: focus and seat. Focus should technically be the easier one, but I can't tell you how many times I intend to ride a transition at every quarter point of the circle and end up either not on my circle line anymore or end up trotting around the whole circle because it started feeling really good. It's much easier if you have your trainer yell at you to WALK NOW! or else. The good news is that you can challenge yourself on that in every ride, all the time.
Now for the seat. Yes, that is a gigantic can of worms. And I'm not going to herd wiggly earthworms around. You can read examinations on how to sit on a horse all day long, but you have to actually sit on a horse with a trainer who can get you to align your body correctly and create the right amount of internal resistance. Yes, internal resistance. It's only then that you can truly push your hands forward instead of pulling backwards on the reins; get maximal influence from your seat bones and get your leg underneath you to use it in a nuanced way that consists of more than "off" and "kick" modes. Over the years I've been told all sorts of things: to stretch up tall, to slouch, to bounce like I'm sitting on a beach ball, to balance forward like I'm on a pogo stick... and I've had my leg pulled into so called correct positions that were so painful that I couldn't keep it there for a second after the trainer let go. But it wasn't until I learned about that internal resistance (from Tanya Vik) that increases the stability of the core dramatically that my riding became much more effective.
So now, when I ask for a transition and the horse offers the pop-up in front, I go right back to making sure I'm in my strong seat, have my legs underneath me, maybe a little flexion, sometimes that's a little outside flexion, and then press with my lower legs for that powerful strike off and reaching for the bit over the back. And then again, and again, and again....