In Fallout 4, you can enter a relationship with as many companions as you like without the fear of repercussions. Before you can enter a relationship with anyone, you have to make sure your affinity level with that companion has reached the maximum level. You can check out our handy guide here on how to raise your relationship with every companion in the game. Each time your affinity level reaches the next threshold, you’ll be able to have a conversation with your companion. He or she will start repeating the same line of dialogue over and over until you talk to them. You’ll then have to pick the ‘romantic’ or ‘flirtatious’ dialogue option to get the love train chugging along.
However, do note that successfully pulling off all of the romantic dialogue options requires a little bit of Charisma and Luck, so it’ll be wise to have at least a few points invested in both of those categories. Even if your Charisma and Luck levels aren’t that high, you can always quicksave and load again until you successfully get past the speech check.