Before you say "I do", make sure you're excited about living with this person for the rest of your life.
He/she is not going to change after marriage.
If it's not working while you're dating, marriage won't make it work better.
If he's a slob at 25, he won't become meticulous after marriage.
If he cheats,...
If he's kind,...
If she's inconsiderate,...
If she's a ambitious,...
Etc.