Because you love to.
At heart, that’s the only reason to do anything. (Of course, we might have to continue to penetrate the meaning of love. But, back on-topic--)
If you’re not so sure that you love to write, please let me continue.
1) Humans “write” because it is our distinctive character. Kenneth Burke, called the most important rhetorician of the 20th century, distinguishes humans as the “the symbol-using, symbol-making, and symbol-misusing animal” (Language as Symbolic Action). If your symbolic action doesn’t correspond with “writing,” consider expanding the field of composition.
2) If you have difficulty claiming that you love to write, explore options. Troy Hicks uses the acronym MMAPS to guide teaching of comp, digital or otherwise. Look at the mode, media, audience, purpose, and situation that you use and play with them. I discovered joy in composing as I played with digital media. I got into DMP in ought-to mode; but as I tried out production programs that matched up with my disposition, I connected the photography I love with words, found ease of precise editing, and focused on a topic of passion (riding horses). I’ve continued to grow in love with Digital Media Production, but it’s still hard work. (on DMP, check this.)
3) Let writing go to edge of consciousness. That’s where vitality comes in. Like dressage masters, we should always keep in mind that if, at the end of the lesson, the writer is not happier and healthier, then something needs attending. If we don’t write until we find this for ourselves, how can we expect it in our classroom?
3+) If you can’t do it for yourself, do it for the kids. Sorry about pulling that card, but we know that good teachers of writing write, like good teachers of reading read. And we know that the best teachers teach what they love: enthusiasm motivates intrinsic, authentic, life-long learning like nothing else. Check the process-product research; or better yet, check out your own self-directed drive.
4 or 5) Can’t find time? No easy solution here, but try buying yourself out. Explore paying someone to do something you’d rather not do (house cleaning, food prep, walking the dog) or even doing without x (a meal, a TV show, a graded assignment). You might be surprised how hard it is to prioritize what we love, especially the kind of passion that takes time and tending.