I'm just thankful they got rid of the "dislike" button. There is an author I spotted with a rep of something like -16
But yeah, I run out likes all the time. It was really frustrating reading Bill W's Castaway Hotel, because at times I'd read fifteen or twenty chapters in a night, and the annoying little "you have run out of positive votes for the day" message kept on popping up.
I'm not sure why the limit of likes was set to five either, but, like Andrew says, I think it was probably designed to prevent over-inflating an author's rep. But when you get an author on 1000+ rep what's an extra few likes between friends? Besides, on day's when I'm not reading, I tend go back over any chapters I didn't get to like the day before, and like them then.
And it's even more annoying at the moment with the release of the fall anthology (13 stories, 5 likes per day, you do the math I've already run out likes, so the other anthology stories are going to have to wait until tomorrow to be read).
Then we have the GA anniversary anthology due out soon, and since that is a novella anthology we can expect probably 100+ chapters being released at once (depending on how the various authors have set their stories up).