Why can't Ranbir Kapoor and Deepika Padukone be a couple?

Every time the two actors shoot for Imtiaz Ali's Tamasha, its photographs are leaked and consumed with avid curiosity. What is it about the two, seemingly happily in relationships with two other equally good looking and successful people, that we can't get enough of?

#1. No other couple gave as many interviews when they were together: 

We are starved of celebrity romances. When Deepika and Ranbir were together, they were so happy to announce they were, parting with details like what they ate on their first date and what they wore (the magazine they gave their big combined interview too is now defunct as is their relationship). It's light years away from Ranbir and Katrina Kaif's tight-lipped togetherness, and Deepika-Ranveer's are-they-aren't-they status.

#2. There are not too many other relationships in Bollywood that can be talked about openly, or at least not in polite company: 

Married actor with a single actress. Married actor with a star's former trophy wife. A single actor with a fondness for lookalikes of his most famous girlfriend. There's not much to say about Bollywood's less than perfect liaisons.

#3. It was the ultimate North-South union: 

Dharmendra and Hema Malini. Amitabh Bachchan and Rekha. Bollywood loves its legendary cross Vindhyas relationships.

#4. They both have famous parents: 

He is from Bollywood royalty. She is from sporting class. When the twain met, it seemed a match made in heaven. He could teach her acting, she could teach him what to do with a shuttlecock (now, now don't be naughty).

#5. They weren't together long enough: 

They didn't give us enough time to invent a combined brand name, or do ads together, or be spotted holidaying together in secret locations. They were together and then they were only on screen. Their movies so far - Bachna Ae Haseeno (2008); Yeh Jawaani Hai Deewani (2013) and now Tamasha.