“Don't get too attached to anything.”

Source: The Truth About Forever

Source: We Need to Talk About Kevin

Thomas Merton, in New Seeds of Contemplation (1961)
Misattributed

Music Preservation Society biography http://www.wchandymusicfestival.org/downloads/HandyBiography.pdf
Variant: Life is like a trumpet - if you don't put anything into it, you don't get anything out of it.

Context: To enjoy anything, we cannot be attached to it. William Blake understood this beautifully: He who binds to himself a Joy, Doth the winged life destroy; But he who kisses the Joy as it flies / Lives in Eternity's sunrise. What we usually try to do is capture any joy that comes our way before it can escape. We have our butterfly net and go after the joy like a hunter stalking his prey. We hide and wait, pounce on it, catch it, and take it home to put on our wall. When our friends come to visit, we say, "Hey, Stu, would you like to see my joy?" There it is on the wall - dead. We try to cling to pleasure, but all we succeed in doing is making ourselves frustrated because, whatever it promises, pleasure simply cannot last. But if I am willing to kiss the joy as it flies, I say, 'Yes, this moment is beautiful. I won't grab it. I'll let it go.'

Sam Harris, Big Think Sam Harris On Death http://bigthink.com/ideas/3127 (July 4, 2007)
2000s
Context: We just don’t teach people how to grieve. You know, religion is the epitome, the antithesis of teaching your children how to grieve. You tell your child that, “Grandma is in heaven”, and there’s nothing to be sad about. That’s religion. It would be better to equip your child for the reality of this life, which is, you know, we... death is a fact. And we don’t know what happens after death. And I’m not pretending to know that you get a dial tone after death. I don’t know what happens after the physical brain dies. I don’t know what the relationship between consciousness and the physical world is. I don’t think anyone does know. Now I think there are many reasons to be doubtful of naïve conceptions about the soul, and about this idea that you could just migrate to a better place after death. But I simply don’t know about what... I don’t know what I believe about death. And I don’t think it’s necessary to know in order to live as sanely and ethically and happily as possible. I don’t think you get... You don't get anything worth getting by pretending to know things you don't know.