It's the default on reddit because you should like your own content in almost all cases.

I do it too, though I'll then change it to a downvote if I say something which is later demonstrated to be wrong.

10A, we meet again. yes, i do it cause im use to seeing +1 upvote on reddit, and posts look sad and lonely starting off with 0 and 0 lol. cant say i ever downvoted my own post or comment tho, usually ill edit it and correct myself in that case, and not upvote it. everyone has their own way of going about it (ㅅ´ ˘ `)