First of all, the movie makes it clear that lying has a way of hurting both others and yourself.
Maybe you don't consider it a lie when you make a promise and you can't follow through because of circumstances beyond your control, but to the person on the other side, it still feels the same. You said you'd do one thing, and you didn't keep your word. Whether it was your fault or not, that doesn't always change things - especially not in Fletcher's case, where we learn that this is a long-standing pattern. After Fletcher fails to come through too many times, his ex finally decides she can't put herself or their son Max through this anymore, so she accepts an offer to move away and marry her boyfriend. She's not really ready for that step, but she has to do something to make a change.
Lies and broken promises will hurt the ones you love, and they will wind up hurting you, too! That's why the Bible says we should never lie, and it's better to not to make a promise at all than to make one you can't keep (Leviticus 19:11-12; Ecclesiastes 5:4-6).
"Lying lips are an abomination to the Lord,
but those who act faithfully are his delight."
- Proverbs 12:22
Life would be so much easier if we all just told the truth all the time, then, right?
Well, maybe not...
This is the second big truth the movie shows us: Telling the truth can still hurt others and yourself.