Well, for me I've been thinking about these stories and characters for years! I love drawing and writing so comics works out for me as a mix of both 
I remember completing the comic script for my current comic and feeling so proud of myself. I finally had a complete story written out and I absolutely love all the ideas, events and character moments to the point I still go back and re-read them because I'm so excited to finally illustrate them one day!
For me, I want to one day be able to hold my comic as a printed volume, and read my own series. I write about things that interest me so having something that I know I made really makes me happy. My motivation is mainly because I love doing it. Seeing my writing come to life through my art is insanely cool.
I do also get motivated by other people enjoying the comic as well. Having my readers notice things about the story that I've planned and psychoanalysing my characters makes me happy because it means my intentions are clear to the readers, and they are interested enough to pay attention to details. Having people love my comic is definitely a big motivator, but it's not my only one.