With all due respect, it's my opinion:
Everyone has different theory of love. Not everyone's opinion matches.
Love at first sight does exist. It grows stronger when two people get to know eachother.
See, love at first sight doesn't necessarily mean falling for looks. There are things called Telepathy, Intuition & positive vibes. When you see a person for first time you might want to know the person or you might be attracted to the person.
Now, if it's only for beauty, then many people wouldn't have tried to persue someone they saw for first time, who isn't very beautiful or the very definition of beautiful. It's also about vibes you know..
Also, in Ruhaan's case, if he were to fall just for beautyhe would have already had a girlfriend. Because, he is a fashion photographer & he would have met much more beautiful girls than Mishti...
So, it's not at all illogical.