Why do some men propose when they lose the girl?

This happened to one of my co-workers. She's now married for 4 years but it was not until losing her (in fact she was on the process of booking a flight reservation to another state), he chased after her with a ring. My own male cousin tried that too but his ex girlfriend lost total interest in him and declined it.

Why do some men wait till the last moment when she leaves? I always thought a proposal should a happy moment in the relationship, not when trying to fix things.

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Why didn't they put that desperation, that dedication, that ''omg it's now or never'' while they still had her? Why do that when she's gone?'
Why do some men propose when they lose the girl?
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