I cannot tell you how many times I've been asked this question. And to be honest, sometimes I am the one asking it. Not in an "I don't want to to this anymore" way, but in an "I always want to remember the why because it helps get us through the tough stuff" way.
Foster care is hard. I will not tell anyone it is easy because that would be a lie. But it isn't just hard for the foster family. It is hard for the child who has been taken from everything they've known. It is hard for the birth parent who knows their child is with a stranger, in a strange home, and who has heard horror stories about how terrible foster parents can be to the children placed with them. And yes, it is hard for the foster families who have to change schedules, add appointments, open their hearts to love a child who won't be with them forever, get to a know a new person, and still give their own children attention (all without much notice at all).
So why would anyone want to subject themselves to all of that brokenness, pain, inconvenience, and upheaval? There are many answers to this question, but it all boils down to the fact that it is worth it. Does it always seem worth it in the moment? No. But then something brings you back to your "why". It can be something like a baby crying because they are hungry. Normal right? Not for a baby who has learned that crying doesn't do anything. It can be something like a child hugging you for the first time, weeks after they moved in with you because they realize you are safe. It can be a child's grade going from an E to a C because they can finally concentrate on something other than their safety.
Those things motivate us but God's calling on our life motivates us even more than those things. We are called to foster care which means we are called to love, bring freedom, and show the Father's heart to each child and to each birth family. We are called to be vulnerable and willingly give our hearts over and over again even though our hearts will break every time we say goodbye. It is only because of His love that we can love. It is only because of His faithfulness that we can trust Him to care for them when they aren't a part of our lives anymore. It because of Him and His love for people that we do foster care. He is our "why".