Can you give me some verses that say on what basis God chooses? I know He chose David because of the condition of his heart.
He chose Saul for who knows why, maybe to show israelites they should of listened to God and kept the judge system God gave em, but Israel insisted.
He chose Jeremiah and Paul before they were even born, that is a hard one to explain.
I would suggest that God loves and chooses ALL conceived human beings for salvation, but not all human beings choose to love God back and trust Him, so pass up on the salvation He has chosen them for.
God chooses and prepares some particular people from the womb for some particular roles to course-direct the world or a nation toward His non-negotiable goals. He probably had a few babies contemporaneous with Jeremiah who could have done what Jeremiah was commissioned and used to do, but Jeremiah is the one who first said,
"Here I am, Lord. Yes, I will go for you."