It's growing up to the point that you're willing to tolerate the loss of a beloved, irreplaceable partner. There's only one way to avoid that inevitable loss, and many people follow that strategy - don't love your partner too much, so that by the time they die, they won't mean that much to you anymore. Like many aspects of life, the choice is not whether or not you're willing to be anxious. The choice is which anxiety you're going to have, and you're going to have one of them, whether you like it or not. You can lose your partner a little every day, or all at once.
Nobody ever gets to have a better partner than they're willing to give up.
Willingly loving someone, knowingly setting yourself up for loss - that's hard for people. It goes beyond resolving sex problems or personal insecurities. It's an issue of integrity. Relationships are a people-growing machine. They pull and push you.