It depends on lots of things; everyone's different. The main factor is where on the ABDl (adult baby-diaper lover) spectrum you are and even then there's different aspects to it all. Being an AB can be about getting treated like a baby in every respect; being taken care of, being fed, changed etc. Some like to do it voluntarily, having a 'parent' to love them and so on. Others enjoy being forced into such situations, being powerless, trapped, forced to use their diapers and depend on the parent figure to do everything for them. Being a DL is just about the diapers but it also carries the same two varieties. The other main factor is whether or not it is a sexual thing.
I think, however, this image, and the others in the 'more like this' box, are more about the cuteness than the ABDl.
"My theory is it has something to do with being forced to feel young again/The idea of someone taking care of you in ALL aspects. But I could be completely off" Nope, you're spot on. That's the ABDL fetish. Like other people mentioned, there's a bit of a spectrum, but that describes the brunt end of it.