I don't want to be disrespectful, but I remember the story of when Rabbi Akiva saw the temple destroyed with foxes walking where the Kodesh HaKodashim used to be. He laughed because even though everybody saw the bad side, he understood that because of this the Third Temple should come. So why in times of Tisha B'Av we cry? Shouldn't we see the good side even in the worst of tragedies like Rabbi Akiva did?
Interesting question. However I think before asking that we must ask ouselves if we cry that there is a question whether we should laugh instead? We are so deep into this galus that the few things we do to try to remember the beis hamikdash are so hard for us to keep and the one day a year we spend on trying to tear at least a drop, to many is so hard to relate to. Rabbi Akiva didn't have to force his laugh just as the other tanaim didn't have to force their cries. Those were real emotions. The tanaim who saw the destruction genuinely cried as they felt Hashem 's pain and realized klal yisroel 's errors. Rabbi Akiva saw the other aspect and laughed out of rejoice of the emes of or Torah and neviim and that although sometimes we as individuals or as a nation fall, ultimately we are bonim la'makom and will always be loved by Hashem and will ultimately be able to become closer to Him than we could imagine by Binyan bais shlishi (the rebuilding of the third temple ). It is not a question of whether we should force a cry or force a laugh, it is a challange to 1. Understand the two emotions and then 2. Depending on our level channel it based on our perspective. However if we cannot come to cry from seeing and feeling and internalizing the destruction of the beis hamikdash then we are already a step behind rabbi akiva who first fully understood that and only then followed it by, "if the beis hamikdash was destroyed then certainly another will in the future come to be rebuilt!"