I believe Mac is talking about a significant other leaving a relationship, as they believe they're missing out on other things, and may never get another chance to experience them.
The grass is always greener type deal, Mac implies they will regret the decision, as things were never really that bad, and once the person goes out and tries to gain these experiences, they will realise that leaving was the mistake.
But it's too late, they've made their decision. They're out.
I believe Mac is talking about a significant other leaving a relationship, as they believe they're missing out on other things, and may never get another chance to experience them. The grass is always greener type deal, Mac implies they will regret the decision, as things were never really that bad, and once the person goes out and tries to gain these experiences, they will realise that leaving was the mistake.
But it's too late, they've made their decision. They're out.
Don't take your relationships for granted guys!