I have two comlpletly different answers to the riddle. The logic will be very strange in both explanations. I also know both are probably wrong, but I tried.
First Answer is:
Act, as a verb means doing something. From a certain point of view, no act last forever, it will be lost in time at a some point. But sound depressing.
Act as a noun by the definition of a dictionary is
the doing of a thing .
Also there are at least two derived nouns which partly fit the riddle. Activity and action.
A certain action or a certain act, or even a certain activity at a certain time can lead to the described situation.
Again the logic is a bit far-fetched here.
Act as an adjective is also not used, but active does exist.
Now here is the most far-fetched thing.... Acive means working, doing, happenning, ect so going by that, if you don't believe what's happening right before your eyes, then you can be laughed at.
A big thank you to @Quark-epoch for pointing out that
Act can be used as a noun
A big thank you to @Omega Krypton for pointing out that
Act can also refer to a story. It makes Act as a noun fit as an answer better.
Second Answer is:
Life as a verb is Live, and it will sound even more depressing, but it ends at some point in time.
Life as a noun is ever changing. In life anything can happen. Also sounds a bit depressing.
Life as an adjective in the simlest examples is lively and alive.
Again, lively doesn't fit the riddle well.
Alive, however fits to a certain point. If you don't belive you are alive, that can be depressing. But the life is still there.
As I said, the logic behind these answers is very strange and downright depressing, but I have nothing more on me right now. I am still trying for solutions.