To romanticize is either to put a positive spin on something that wasn’t great or to behave in a romantic way. If you’re sea sick on a cruise and spend the whole time clutching the railing but later say it was the best trip ever, you have romanticized your experience.
To romanticize can mean to act in a way that lends itself to romance, such as by buying flowers or making goo-goo eyes. A more common meaning has less to do with romance, but also involves wearing rose-colored glasses. If you romanticize war, you're making it sound like a glorious, beautiful thing. To romanticize is to interpret things that are not glamorous in a glamorous way.