Imagine that everyone’s self lives inside the house of their body.
Most people are pretty content with the fundamental structure of their body, as it matches a blueprint they have in their heads. It might need touchups here and there, or small changes, but nothing fundamental.
A trans person, on the other hand, has a blueprint for a completely different type of house than the one they live in. They might try to make the best of it, especially if the “house” they’re in is nice and functional. But they’re never going to be as fulfilled as if they lived in the right kind of building.
So there’s a longing there, that can get worse as time goes on.
Transition would be the major renovations that made their house actually look like the blueprint in their heads, which cannot be changed. And let them get on with just living and using the house like other people do, instead of constantly being distracted by the wrongness.