Why do we often put off saying things we think and feel until it's too late?

It's called "confession". In the church it is anonymous on both sides. At a funeral there is no response. Try to do that with a live person and you get a nervous snappish reply. The general attitude is "Don't tell your problems to me!"

maybe cause we dont know we wont get another chance to say it later

There are many reasons.   

Sometimes we think it's assumed (they already know it so why should I say it?)  This is often the case with family members.  Do we really tell they we love them or appreciate them, or do we simply assume they know and let it go at that?  

Sometimes we think that if we say it we might be misconstrued.   That could cause a strain or rift in the relationship.  So we  remain silent... just in case.

Sometimes we don't want to offend the other person.  Offending someone can also put a strain on relationships.  While some relationship can handle it, some can't... so why take the chance?

Sometimes we're simply shy.  Not everyone feels comfortable sharing or saying personal things to others. It's not necessarily an introvert/extrovert thing either.


Please note that when someone whispers something to a corpse, they're not necessarily saying something they failed to say while the person was still alive.  Saying goodbye to an elderly person each night just in case they die is rather morbid (and rude).  But saying goodbye to a corpse can help bring closure.  Saying "I love you" to a body one last time isn't a regret.  Death doesn't stop love. 
Nearly everybody has a regret when someone dies (I should have said this or did that).  Death comes so abruptly that there'll always be one more thing we could have said or done that we didn't - no matter how much we communicate with the person (say or do).

Sometimes it's because what we feel is a crime. I saw my high school principal fifteen years after graduation and I really wanted to go punch him in the nose. I mulled it over and decided he was not worth the fuss it would cause, so I let it go. To this day I wish I had done it, but I have to admit I made the wiser choice. But I still wish I had done it.