why?
because 大卫 knew WHO would say the FINAL words
concerning who lives and/or who die!
thus,
he desperately repent and ask God for mercy to him and to Bashiba,
God didn't grant any that time,
大卫 accepted the fact and proclaimed that I will join this little boy
one day, this proclaimation has been treated as a promise for those who
died prematurely.

大卫是合祂心意的人的原因, in my humble opinion, is
that
he had been doing his best to please God,
one occasion, he messed up, he took all the responsibility
and repent desperately with action, from this perspective
he is a role model!