I think it was place names that finally tipped me over the edge into learning Welsh. It was the realisation that the names had a history or described the place - and wondering whether 'Nant y Caws' really meant the 'Stream of Cheese'.
I didn't do well with languages at school, so learning Welsh has been something of a revelation, that not only can I learn another language, but that learning another language isn't about saying the same thing using different words. There's an entire other dimension of thought that opens up as soon as you have more than one language available - even if it's only a bit of another language. There's no longer the equation language=english. Language becomes a variable rather than remaining a constant. You an choose how to speak or think The feeling and tone in the mind and on the tongue is different between different languages.
Culture and history come with the language. I've learned things about Wales that I should have learned at school, but which were sadly absent from the curriculum. I've also discovered a wealth of music and poetry that I would otherwise not have encountered.
It has been good to be able to talk with people in Welsh. The ability to have even a simple conversation has been a revelation.
So what started me learning Welsh may have been an interest in place names. What keeps me learning Welsh is the sheer joy and fascination with language, art, poetry, music, history and culture and not knowing what other aspects of life are yet to open up as a result of learning.