This may sound a bit strange to you (especially guys), but while learning how to play a guitar, the single most motivating thing for me was: Singing I know, I know...
you don't think you WANT to sing.
Or you don't think you can.
But you CAN.
I don't even have to tell you that there are plenty of professional musicians that sing terrible *cough*James Blunt*cough*, and people actually like to listen to them anyways! Literally, you can become a one man (woman) band.
You're guitar carries the rhythm/beat and you sing the melody.
It can be greatly satisfying while you are alone and, once you get good at it, you bring a WHOLE lot more to the table when playing together with others than the average guitar player (and there are a lot of them).
Just practice somewhere you can experiment and focus on singing loud.
That may sound counter-intuitive when you are first starting out, but you will learn to control your voice far faster by singing "out" and "loud".
Sing "at" a wall.
Pushing the voice out of your head.
If you start to feel your lips or teeth vibrate just a speck, you've scored! Once you can sing a song at that volume you will be well on your way to being valuable to others for your added vocal abilities.
Do take it easy though, your voice is just as much an instrument as your guitar and shouldn't be abused or taken for granted.
Have fun with it but show it respect!