Samson had found love. He wanted to spend the rest of his life with this woman. She made him feel so happy. What could be so wrong with acting on those feelings of love?
It went directly against the Word of God. 
Samson had been set aside since birth to do the work of The Lord. He knew the laws. He knew that Jews were to only marry Jews. Yet he fell in love with a Philistine woman. Then he followed his heart and took her as his wife. She would eventually betray him.
As Christians we can get into trouble when we base our decisions on our feelings and not on the Truth. We have to be willing to seek God’s way and then apply it to our lives. Even if it doesn’t make us feel good.
Just because we want something to be found acceptable by The Lord doesn’t mean that it will be.
Judges Chapters 14:1-7