Don’t base them exclusively on physical and/or sexual attraction. While these kinds of attraction can certainly strengthen relationships, they can’t create anything but a weak foundation for a relationship on their own.
Know how your characters like to show and be shown affection. Not everyone shows their interest in others the same way. Some people like to give gifts. Others like to cuddle. Still others like giving compliments. Different people like to receive different kinds of affection as well.
Remember that love at first sight is a myth. You can have lust at first sight and romantic interest at first sight, but true love takes time to develop.
Show the characters interacting and getting to know each other. This should be obvious, but it is all to common for a character to be given a love interest at the last minute or to be paired off with someone the reader hasn’t seen them interact with much. Remember, the reader doesn’t have to see every little thing they do together, but the relationship will feel forced to the reader if they don’t see the characters interacting and establishing that they genuinely care about each other in a significant way. If the reader views your character’s significant other as little more than a stranger, then you’re doing something wrong.
Have both characters do things for each other and contribute to the relationship in meaningful ways. Relationships are two way streets. While you don’t need to keep score of exactly who does what for who (Relationships are not a competitive sport!), the relationship should seem fairly balanced or, if it’s not, then the characters should be working to change that.
Don’t give your characters completely incompatible traits. While it’s healthy for people to differ from each other, there are some differences that even people that are otherwise perfect for each other probably can’t overcome. For example, a environmental activist would have a hard time having a healthy relationship with someone who wants to chop dow a forest. Basically, know your characters’ deal breakers so that you won’t try to match up characters who are simply incompatible with each other.
Have them share interests. This is a great way to add substance to relationships outside of physical attraction and compatible personalities. Maybe they both like fishing. Maybe they share a passion for baking. Whatever you decide to have them like, don’t be afraid to use your characters’ shared interests as opportunities for them to bond. Also, if your characters don’t share a lot of interests/hobbies, consider having one character introduce the other to their hobby or have one character take initiative to try something the other likes. This is a great way to show how much your characters care about each other because it demonstrates your characters’ genuine interest in what makes their partner happy.
Let the relationship experience at least a few bumps in the road. No relationships are perfect. Let your characters disagree, argue, and maybe even have a full on fight. Relationships that withstand obstacles seem stronger to readers, especially if the characters grow as people because of these hardships.