Why did Bharat refuse Raina’s marriage proposal?

In Bharat, the titular character falls in love with Kumud Raina (“Miss Sir”), who proposes to him publicly. However, Bharat refuses and instead they have what he describes as “probably the first live-in relationship in India in which the mother approves”.

I don’t understand though: why didn’t Bharat accept her proposal? It would be more socially acceptable in the 1970s, it was what she wanted, and I don’t see any way in which it would impede Bharat’s life goal to keep Hind Ration Store open in the hopes of finding his father.

Similarly, in the final scene, the two decide after forty years of living together to finally get married. What changed? The only thing I can see is that Bharat accepted that he will not find his dad and that the shop will be destroyed to make room for the mall. But how does that in any way affect his decision to get married? Did something else happen?

So why didn’t Bharat accept Raina’s marriage proposal? And why did he change his mind 40 years later?