LP MEAN NOTHING!
That's the first and most important thing to remember/know!
MMR is the one and only thing you should ever care for.
MMR decides the players you're matched with.
MMR decides the players you're macthed against.
All this messy LP system does is comparing your MMR to the avarge MMR of the Division you're in!
If it's higher you gain more LP and losse less.
If it's lower you loose more LP and gain less.
All for the sake of keeping your League as closely as possible to your MMR.
Obviously whenever you climb a Tier your LP lowers for reasons I just mentioned.
But if you keep playing 'normaly' you will reach a balanced LP gain/loss eventually.
Alternatively:
If you're really pissed of by those LP gains you can hard fix your MMR:
If you reach a promo you can dodg-cancel your series after winning one round to artificially boost your MMR in comparison to your Division since you're no longer cimbing in Ranks will still gaining more and more MMR. (requires a positive winrate.. obviously)