So according to an IF member, a conversation happened between Upen and Karishma
"Karishma questions Upen, as to why he couldn't wait till she came out of the house? And why was he so desperate to come back inside? Further she asked him, what was the need to create such a drama amidst everyone, in the Bigg Boss house? To which he replied, that he was not sure if she would even meet him, once she was out and that she would get busy with her life and friends. Upen added that he knows she was equally desperate to convey her fondness and affection for him since before he was evicted from the house."
No doubt, this second stint has helped Upen in his image correction, plus some money for a vacation, but the way their scenes pan out, the understanding between them, the attraction doesn't seem unreal(except those cheesy and OTT scenes in which they rate each other). The tension and closeness between them was very evident in the last few days before Upen's eviction. The way he would unnecessarily insult Sonali only infront of Karishma, the way he would protect her infront of Salman, his possessiveness, etc was all indicating towards a relation bigger than friendship.
Even Mahek has pointed out that Upen told her on Finale shoot to take care of Karishma and sent in a lot of stuff for her. So, it was clear he was head over heels for her plus he was insecure/unsure about her state of mind after her return.
Seeing the response and support KT is receiving outside, he would have rightly felt that KT might not acknowledge and reciprocate. Once she's out, she might get busy with her assignments, projects and life in general and will friend-zone him. If she can forget her bf in 3 months with him, she might come back to her old self in a month without him. So, I think it was a desperate last resort attempt on his part to woo the girl. He knows her inside out, all her good and bad points plus he has been single for last 3 years, probably looking for the right girl. And I think KT comes closest to what he would have wanted. How else can you really find that right partner than at the bIgg boss's house where you are holed up with others 24 hrs 3 months.
Hence, the only way to make her understand the depth of his intentions and to make her feel secure in this new relationship, he went all out in openly displaying affection to convince her. It was certainly a selfish move but then he was looking at the bigger picture, probably a companionship for life. Of course he ended up ruining KT's image in this process but he also fetched her the much needed limelight.
Without Upen, KT was like an unguided missile, very headstrong but also very stubborn. Upen gave her the much needed outside perspective about how she has to mould herself to what the public really likes. He was able to bring out the softer side of Karishma out. SOme would continue to point at her infidelity and refuse to look at her other attributes, but lets face the truth she has shown all the colors in this Color TV show. Drama, romance, tears, fights, performance in tasks, determination. Like all human beings, she has her faults and priorities. But what's surprising is the way she puts everything behind her and continues her journey ahead.
I wonder, if KT loses in the finale, how she'll react to all the name calling owing to her PDA towards Upen in the house and her ditching Rushabh. Or she might see the bigger picture like Upen and carry on the path she has chosen for her with Upen. Because as they say, everything is fair in love and war.