Married love must be the FREE choice of each spouse. Each member of the couple needs to be freely choosing the other person; they cannot feel like they have to get married.
When you imagine a lack of freedom, your mind probably jumps to being forced into marriage against your will. In the olden days, the instances of forced marriages were much higher than in modern day USA. Maybe a woman’s parents sold her for a dowry or a conquering nation’s prince married the princess as part of the conquest. Or perhaps some big beast of a man locked a girl’s father up in a dungeon and she exchanged her life for his and then slowly over time fell in love with her captor. Oh wait! Sorry. That’s Beauty and the Beast! ;)
Nowadays in the US, impediments to freedom typically don’t manifest in those ways. And thank God for that! Even though nobody was probably kidnapped, obstructions to a couple’s freedom still exist today. One example of a Church-acknowledged impediment to freedom is a pregnancy - if the woman is pregnant, there is a possibility that the couple feels obligated to marry (because their parents want them to, because they think it’ll be easier to raise a baby if they are married, because that’s just what you do when you get pregnant, etc.). Even if the couple was intending to get married one day and then conceived before that happened, being pregnant at the time of the engagement still poses a potential impediment to the uninhibited freedom of the couple. There may be a pressure that wasn’t there before.
The Church wants to help people ensure that they are giving a totally free “yes!” on their wedding day. In order to do this, after you become engaged, you will have a meeting with the priest where he will conduct what is called the canonical interview. One of its purposes is to ensure that each individual is free to enter into the marriage. The priest will separately ask each partner a series of questions to make sure both people are entering into the marriage of their own free will.
If you feel obligated to marry your fiance for some reason and you are not free to say no, then you are also not free to say yes.