Do this person and I want the same things?
Make sure that before you get engaged, you've talked about your long-term vision for life together. A classic deal-breaker is that one person wants kids while the other doesn't. But other plans matter just as much: is your partner going to demand that you're home by 5:30 every day, whereas you want to hustle and grind to advance your career? That could also be a serious deal-breaker.