How to generate a plot when the love interest is a good guy, and therefore she needs to neither reform him nor lose her common sense to have a happy ending:
It does not have to be a romantic rivalry; the Mad Scientist's Beautiful Daughter often feels conflicting loyalties between the hero and her father, and has to learn the extent of his evil before she is willing to support the hero whole-heartedly.
He may be nice, but an idiot, or have some other character flaw that the heroine has to get past if she's going to be with him. Often leads to Tsundere behavior.
They face no internal problems, but external forces try to tear them apart. Star-Crossed Lovers frequently fall under this, as does It's Not You, It's My Enemies. This was particularly popular in the 1700s when romantic comedies were typically not stories about falling in love, but stories about getting married without being written out of the will.
The hero or heroine (or both) face difficulties in admitting their attraction. One of them might even be in a preexisting relationship — and since he's such a nice guy, he's not going to stomp on his current girlfriend's heart or go after someone who's already taken.
There may be some romantically unrelated obstacle to their relationship. The two of them are effectively "together" but they can't take that last step until this obstacle is removed. For instance, there's a war going on and one of both of them are fighting in it, or he is Unable to Support a Wife and thus seeking a fortune. Maybe there's an Engagement Challenge of some sort.
Maybe the conflict comes from turns out to be one is a straight-laced hero and other is an Anti-Villain of some sort but the protagonist or love interest is either initially unaware or already knew from the start. And while one might think this would lead to All Girls Want Bad Boys (or Bad Girls), the opposite happens: The hero may disagree with the other's way of doing things but also admires their more noble qualities; and the "villain" also disagrees with the hero's way of doing things (possibly from dealing with fake heroes in the past) but falls for the hero's genuine and honest desire to do good. And as stated, the conflict comes from falling for the other's goodness but also believe that their own respective way of handling things is the right way. It's a complicated case of enemies to lovers.
Archetypal Characters, other tropes, and their usual characteristics of the former two that explain reason why someone would seek a good person as a romantic partner
All-Loving Hero = While most forms of media tend to underestimate a kind and forgiving soul, can warm the heart of someone who initially had no optimism left because of tragic circumstance. Being with someone who is genuinely kind, friendly, with an innate desire to help people; being around someone who gives off such positive vibes can make them very attractive to certain forlorn individuals.
Chivalrous Pervert/Lovable Sex Maniac = While the love interest may find the character's sexual libido to be a dealbreaker, they still have their more positive traits. Perhaps, the hero or love interest, in a Moment of Weakness, wants to have Sex for Solace; but the aforementioned archetype(s) refuse because they don't want to take advantage and state so as much. And they could also display that despite their love for sexual activities, they value consent and listening to their partner's wishes. And if the hero or love interest were to want to try something kinky and be initially ashamed of it, the other person will reassure that there's no shame in having a particular desire.
The Good King/Royals Who Actually Do Something = In most works of media, anyone of royalty is portrayed as corrupt, cowardly Jerkasses. Possibly the heroine/hero or love interest has been the victims of those types of royals and now see all of them like, including their love interest. At first. Over time, they slowly see that the kind royal is actually a fair and wise ruler, actively trying to help their citizens.
Good Parents = Maybe the person in question is a single parent raising a child or children; and one key attractive trait the hero/heroine or love interest might like is just how diligent they are at parenting. Showing fairness, devotion, affection, and attentiveness to their child(ren) on a daily basis. If they have those traits in rearing offspring, there's a very high chance of them being that way in a romantic sense.
Humble Hero = Having an arrogant lover may not always work out, especially if they let their ego go to their head. Meaning possible troubles in the relationship, if said lover can't even admit when they are wrong or need help. In contrast, having a more down-to-earth lover with the humility to know when they are right or wrong leads to a healthier romance.
Knight in Shining Armor = As exactly what the trope states. Some may find said trope to be corny given how overused it may be, but there's no denying its appeal - A noble individual who willingly fights for and protects the weak and defenseless. Anyone would admire such bravery and selflessness.
Nice Guy = Very simple reason. They are just a fundamentally good person, even with their flaws. And those said flaws either are small enough that the love interest isn't bothered, or they gradually grow out of it into an even better.
Understanding Boyfriend = Or girlfriend. Either way, having an emotionally supportive and intelligent partner who just gets you can make another feel truly understood and listened to.
