And also I mean it's pretty much established through the many Hindi Drama staples that SaHil are The "IT" Couple. And I mean, I like 'em too.
But Rehaan, oh man where do I even begin. Rehaan is a character with so much potential, an orphan who never knew love and thus never expected it, a golden-hearted gentleman who never lets his feelings surface because he believes he's not allowed to, he follows his duty religiously because he feels indebted. But he deserves to be shown that there is enough love in the world for him too, that he is just as deserving of happiness as any other human being, and perhaps even more because of his beautiful heart. He's not indebted. He needs to have his firsts in life, unbound by the shackles of his orphan identity. He's proves that grey characters don't have to be total jerks making him so interesting to see.
TL;DR: Rehaan is the orphan who is, the Shahrukh Khan from Swades who feels indebted and restricted, the Banjaara discussed in the song "Banjaare ko Ghar" (from Ek Villain) who gives shade to everyone despite burning in the hot sun himself, the Banjaara who's questions the iraada of his dil when he feels the strange tinglings we know as love. A great character who defies the "Grey Character-Asshole Rule" with a lot of potential, and with regards to his love-story; he deserves his own story on how HE discovers love.

And it's not like I don't want SaHil to happen, I really do.
I just don't want Rehaan to fall in love with Sanam because...
Only two things will come out of it:
1.) Rehaan (a wonderful character who has so much potential to be so much deeper than the single dimension given to him on the show) will be USED as a catalyst for the SaHil love story. Used. A character of his kind played by an actor of Shehzaad Sheikh's caliber (who despite being a newbie is better than a lot of "seasoned" actors I've seen in other shows). And then what? Then he'll back out because he has a golden heart. And then what? He'll be forgotten by the writers. "Ab uski koi zaroorat nahi hai, so whatever."
We won't even get to see his side of the story. They'll make the sacrifice seem like it was no biggie, like KJo did with Aman in Kuch Kuch Hota Hain. We won't get to see how Rehaan's coping after the sacrifice and especially how difficult it was to make the decision in the first place. No emotional compensation on his side. Bas ek use and throw character ban ke reh jayega.
And you know what's gonna piss me off the most, the possibility that they might bring in a half-baked "love interest" for Rehaan to completely minimize the heartbreak that he went through when he sacrificed his love. 3 trip-fall-catches and maybe a few "look-look-hawa-hawa" scenes and wola, he's in love with this new chick justified through a half-assed attempt at development. "Itne paise main itna hi milta hai."
I've seen this happen way too many times to even bring myself to believe it'll be any different. Especially considering it's 4Lions who are great but they follow the rulebook of cliches like its the Bhagwat Gita.
TL;DR: Rehaan will be used. Then he'll will be thrown away. Or will become the "Akash" (IPKKND reference) of Qubool Hai aka he'll just be there to give a shocked reaction in group scenes.
and
You don't deserve to have your feelings used to serve as a freakin catalyst.
2. The above will happen (the USED part) and then he'll turn negative. I don't know if any of you've seen Kaisa Yeh Pyaar Hai but remember Prithvi? I thought he was a great guy and all, helping out Kripa when she was pregnant and Angad left her for like months, and then what did they do to him? Butcher him to the point where I couldn't even stand watching him on screen for a second. They had Prithvi rape her or something, and then had Kripa get pregnant with HIS child, and then made him go insane and like obsess over Kripa. It was just really bad to say the least.