A relationship is like sand in your hand; If you hold it loosely in the palm of your hand, it stays there, but if you squeeze too tightly, it slips through your fingers, amirite?

unless it's wet sand.

Sand is not a very good example.

I would say "A relationship is like water in your hand, if you hold it gently, it will slowly slip away, but if you squeeze it too tightly, not a single drop will stay in your hand."

Now... if only I have a cup...

"The more you tighten your grip, Tarkin, the more star systems will slip through your fingers." - Princess Leia

It's sad that this was the first thing I thought of, amirite?

you should say holding the sand gently, not loosely. Because holding a relationship "loosely" makes it sound like you don't really care much about it.