You add mostest at the end if I love you to signify you love your partner the mostest of all. Whoever says I love you mostest automatically loves the other partner more than he or she does forever and HAS to accept.
This means that Lauren loves Stoney more than he could ever thinks he loves her. She will always love him more because it's just not possible for him to love her more!
When saying "I love you most" to someone after they've said "I love you more" shows that the one sending "I love you most" love the other person more than considered possible
Carrie : I love you.
Frank : I love you more.
Carrie : nope. I love you more, look it up, it's on the internet.
Frank : how about that, you're right, it is on the Internet.