5. She's the reason he got into Elena in the first place.
Stefan and Damon only started going after Elena because of her resemblance to Katherine. Sure, eventually Stefan decided he preferred sweet Elena, but his passion for sexy Katherine started the whole thing. She is the root of his feelings. Elena is always going to be a shadow of that first romantic love.
4. She's already in his head.
Katherine has the power to plant dreams in Stefan's mind. Dreams of them together. Even if they stay PG-13, that's hot. It has potential. Elena sounds kinda boring after spending time with someone as exciting (albeit dangerous and scary) as Katherine. Besides, she's already inspired him to get into bondage.
3. Katherine brings out Stefan's passion.
Let's face it: Stefan and Elena are not the life of the party as a couple. They are too similar. Too "good" for each other. Katherine's crazy, volatile personality brings out the passionate side of Stefan. And his sincere kindness tempers her dark nature. Classic opposites attract. "What is it about you that makes me still care?" he asks, caressing Katherine's skin before they kiss. Then he locks her up. As Katherine noted, there's a thin line between love and hate and these two dance on that line. Did you see him at the Founder’s Day Ball in 1864? He "surprised" her with his declaration: "I've never met a woman quite like you. I look at you and I see an angel. I touch your skin and my entire body ignites. I kiss you and I know that I'm falling in love." That's the kind of love we'd chase for over 140 years too.
2. She is willing to fight for him ... forever.
That Founders Day declaration was a long time ago. But it made a mark. A true love/soul mate/once-in-a-lifetime mark. After all that time, he still has feelings for her and she is still into him. "Don't fight it, Stefan. You loved me once, love me again." Besides, Elena is currently human. Katherine and Stefan are both vampires. Rather than compromise Elena's humanity, just stick with the immortal love you already have. Who cares if you kinda hate her right now. You have hundreds of years to make it up to each other.
1. It would free up Damon and Elena.
Bad boy Damon is balanced out by good girl Elena. His witty snarks always sound best when bounced off disapproving (but secretly turned on) Elena. Without Stefan in the mix, Delena would be free to... fight and break up, make up, and repeat. The relationship would only last for Elena's lifetime — or until he turned her — but it would certainly be memorable.