But secondly, of all the app rejection stories I've ever heard... this one is the easiest to fix. Just add some more features, who cares, and resubmit it.
Ideas: annotate/categorize the things that are making you happy/unhappy. Maybe have categories/icons for them. (Friends, party, loneliness, exercise, ate well, ate bad, had sex, no sex, fought, made up, my team won, etc. -- there really aren't that many fundamental things.) Then also produce charts showing how your happiness correlates with those items.
Heck, you might even be able to produce an amazing academic paper or two with the dataset you produce. Or at least some really cool and fascinating blog entries from the data.
(Personally, I'd rather rate my happiness on a 1-10 scale rather than compare with yesterday, and also maybe be asked 3-4 times a day. Maybe features let you pick those.)