Shahjahan married 7 ladies just to show Mumtaz that out of all my wives you're the One my heart beats for.
Shahjahan killed Mumtaz's husband just to get her, and that's what true love is you can do anything to be with the person you love.
Died while delivering the 14th kid, nothing else can explain it more than this one, that how much they adore each other to have 13 kids and so that she even died while delivering a baby.
Shahjahan married Mumtaz's sister after her death because he missed her so much that eventually he found Mumtaz disguised in her sister's face and ended up tying knots with her again.