I always used to think of tisha b'av as a horrible thing, but then I thought of a new angle. We know that Hashem said "because you cried for no reason I will give you something to cry about." To me this sounds a lot like the concept of worry breaks in anxiety. When you can't stop worrying or feeling guilty ... you can allocate fifteen minutes a day to flood yourself with your bad feelings, and then you STOP for the rest of the day. I wonder if the Jewish nation will always have a need to think that Hashem hates us. So since we can't make it go away Hashem gave us one day a year to let it out. We get it out of our system and move on for the rest of the year.