Although I have never lost my desire, here are a few thoughts:
1. Perhaps you are not getting what you want from aikido, perhaps due to the instruction, school, or curriculum.
2. Perhaps after your enthusiam of finding something new has worn off, your body or mind really wasn't all that enthused afterall. Maybe aikido is not the art for you.
3. Burn out is possible but people generally burn out because they have either put too much into something or they are not getting anything back from their efforts.
I enter the mat every time with the mind set that I'm going to learn something new each time. As an instructor that makes the challenge greater, since I have to learn from my students and my teaching. With my thirst for knowledge, I have never stepped on the mat without learning something-even if it is a small distinction. When I can no longer taken the student mind approach, then I too will perhaps become burned out.