I think a lot of it is because we are here just relaxing and enjoying it. We don't really care if it's not perfect as long as other's can understand it. So it starts with a laziness to some degree. It's much easier to proofread someone else's writing than our own. I agree that the mind knows what the next word is going to be if we wrote it ourselves and it auto-corrects in our mind and we never really see it as it is.
It's nice to have a special talent that you can share with a person you love, like your wife. And even nicer that you take the time from your own work to help her out with her own.
My mom has convinced me that I have very therapeutic hands when I give her a massage. She seemed to think that I healed some lumps she had near her spine that was causing her to walk unevenly. Of course I give God all the glory, no matter how it happened, but it was nice to find a new talent that I could share with a person I love. Was that a trumpet sound as well? lol