Temporarily disconnected...

Is it worth getting involved in something you know will only be temporary? People are constantly advising me to let go of anything that's not going to be there at different points in the future. Pick an increment- a year...2 years...5 years? Will it still be relevant? I mean, in that case, is it worth renting when your heart is set on buying? Is it worth cutting your hair if you know it'll grow back? Should I not wear sunblock since human demise is imminent? Is the immediate gratification that we get from these things lacking a level permanence that disqualifies them from being valued? Even a little? Is it not these things that allow us to breathe, function, and persevere, when we can't seem to reach something seemingly end-less? Isn't that worth something? Or is this our clever way of masking the mild yet fatal doses of disappointment we find rooted in these "for now's"? I guess I just don't know yet.