Like a main character, his or her love interest can tend to run a little flat. Even if the protagonist interests the reader, the love interest may become a burden that simply holds the main character back (while this general happens to a male protagonist and his female love interest, it can happen the other way around). However, love interests should engage the reader and add to the story, not take away from it.
How can you do this?
Give the Love Interest his/her own plot
This character should become as important as (or close too) the protagonist. This means the love interest should have a plot/dreams/hobbies outside of the romantic plot s/he shares with the protagonist. Let the character truly become a person who acts outside of the protagonist’s main plot (or become an integral part of the main plot as something other than a love interest) as well. By doing so, you’ll ensure that the reader does not see this person as an obstacle the protagonist must simply protect/”win”/etc. It will also help the character feel more like a person and less like a stand-in.
This does not mean the love interest needs a POV, just something s/he can do outside of the love story. You don’t have to distort your story and force in some random irreverent plot, but you can let the love interest talk about this plot so the reader’s know s/he has a life outside of the protagonist.
Of course, you can always make both sides of the relationship a POV and main character, like Brandon Sanderson did in Mistborn. If you write Fantasy, as I do, you have the ability to tack on POV’s (if you should and how to manage so many POVs is a topic for another time). So there is no harm in letting the love interest get a few POVs to explore his/her story outside of the protagonist’s. If you want to do that, however, you ought to make sure that plot will connect to the overall story in some way.
Make the love interest as interesting as the protagonist
Don’t plant some stereotype or “set” character type into your story as the love interest. Let him/her be as complicated and unique as the protagonist. Give them faults and strengths, his/her own set of problems, strange quirks, a moral code, and other things that help make any character more compelling. The love interest should be interesting and complex enough for you to write his/her story, if you had too.
Give the couple some common interests
While opposites can attract, the characters should have a believable reason for liking each other (please, please go beyond “s/he was just so beautiful). If a reader is going to “get behind” a couple, the couple has to seem (somewhat) realistic. Give the couple something that connects them (childhood, moral code, hobby, similar political views, etc.). And don’t force a couple. If you planed for a romance that is just falling flat on the page, let it go. Just like in movies, some characters have chemistry. Other’s don’t. If the relationship seems strained to you, it will also seem that way to the reader.
Side tip: show the reader some little moments between the couple. While declarations of love are always great, so are the times where they learn a little more about the other or make each other smile.
Prevent a toxic relationship
Why some people may enjoy a destructive relationship, a lot of people prefer healthy ones where the couple supports each other instead of always tearing the other one down. Instead of having them berate and abuse each other, let the couple help each other grow.
Another tip: let the couple think about the other person when s/he is not around. If the protagonist truly loves someone, he (she) will think about her (him) even when she (he) is not around. This helps show that the love interest is part of his/her life.
If you have any questions, let me know!