The article is a great start (only read up to point 3 or 4). I don't know if it mentioned this, but every relationship is different. I have a story where the boy is really shy and afraid to talk to the one crush he's had for years (since like elementary school I believe). He never talks in class and has only one friend. He admires his crush because she's everything he's not (in his eyes), but she still pretty much keeps to herself.
Another story stars a girl who's been picked on since she was young and has a secret she's afraid will make things worse. She's very anxious around others and has trouble opening up. Her love interest is a loner who will do anything to make her happy and can be himself around only her.
Each of my characters has their own identity outside of the relationship (hope that makes sense). And, each relationship is different, due to outside influences and their own personalities. My characters might be similar in some ways, but they also have very unique personalities.
My advice is to watch other people. Watch how your friends behave around their bf/gfs. Notice the differences in how your friends act, and how each relationship differs. Look for the similarities. Go to a Starbucks or something and just people watch. Notice how some couples cling to each other while others are embarrassed/repulsed by it. Try to notice things like that. The relationship should come naturally imo if your characters are built up enough. Think of how each character would react in a relationship, and what they might be like together. Does the outgoing person suddenly become very shy? Or, how about the shy one becoming the dominant one in the relationship? How are the characters different from each other, and how are they the same?
And don't forget that your characters should still remain true to themselves. Don't let them give up a piece of themselves just to please their partner (unless they're battered and you think it will add to the story somehow, or if you're planning on them breaking up. Just make sure they grow throughout the story.). I hate characters like that, as do many others. This relationship would eventually crumble, and your character wouldn't feel satisfied at all.
If you absolutely cannot come up with anything, then just imagine how you would want your relationship to go. What's your ideal? Your fantasy? How should they woo you, grab your heart? And what little, as well as big, things should they do to keep it?
Hope this helps a little bit! And sorry it's so long...
