Romans 8:28 And we know that all things work together for good to them that love God, to them who are the called according to his purpose.
My opinion is that God didn't cause Samson to fall in love with the woman, but rather used Samson's weakness for His glory. It potentially could have happened differently - Boaz married Ruth, a Moabitess, after Ruth joined with Israel. But Boaz was old when he married, and Samson apparently didn't want to wait? Even the warnings of Samson's parents and the disgrace of taking a wife from the uncircumcised Philistines didn't deter him.
I think with Esther, she likely just was more beautiful than the other women. Humility and a pure heart can shine through as much as physical beauty, and I get the impression (maybe wrong) that a lot of the women on that occasion were focusing moreso on clothes, makeup and outward appearance, than true beauty. Also, Esther's situation was almost a combination of marriage and job application. So again, God was involved in the process, but those who obey God tend to become more beautiful by nature of their obedience - I doubt God overrode King Ahasuerus will.
So lessons to be learned:
1) Don't believe everyone who claims God says you and he are to be together. If that one kills a lion with his bare hands or something similar, perhaps you can evaluate his claims more carefully (although I doubt Samson even used this to convince his Philistine bride).
2) Ensure the focus is kept on obedience to and love of Christ, and don't be afraid to act within that framework - God can close the doors we aren't supposed to open, and keep open those we aren't supposed to close.