If you should single out one specific thing that makes you play the ukulele (and not another instrument) what would that thing be?
For me, it's the SOUND.
Funnunity. (You said just one thing, so I had to make up a new word.)
The uke puts a smile on the face of the people who play it, and the people who hear it. And I have never met a group of people who were more nice, friendly, solid folks, than uke players.
And think about it, when was the last time you saw a listing of Oboe clubs?
I bought it because I wanted something completely different from my guitar, and I got it. Uke is definitely one of the most unique sounding instruments out there, and depending on what you're playing you can get all kinds of emotions into that little body and transform it from the happy little ukulele to a powerful and multifaceted instrument.
I think the uke can be taken seriously...it is just a great instrument, even compared to guitars, and when it catches on even more so than it has people might start taking it more seriously.
No, really - I can't play guitar anymore due to an arm injury, and was getting very depressed at not being able to make music anymore. Then along came this little thing, and I could make music again - bye, bye blues! Why play ukulele? because I CAN play ukulele!
(And even if I could play guitar again, like I used to, I wouldn't give up my little friends!)