He loves and cares for his family but do they give him the kind of love he needs. His Daadi loves him but she loves him as he is her grandson. His bonding with his sister was good but non-ML Abhi found out how venomous she can become if he stops entertaining her needs. Tanu and him didn't have a deep relationship on an emotional level. But, Fuggy was someone who loved him along with his baggage. She loved him when he thought he was being worst to her. He hadn't received that kind of love from anyone. Has he? For a man who was used to giving love to loved ones, receiving love from an unexpected person must had been overwhelming. She was the one who loved him for who he was. That's why he was happy at the end during Lonavala. He couldn't be angry with her for hiding the truth from him or for doing drama, because he was relieved/extremely happy that he got her love back. Her love is like one of his biggest possessions in life.
Coming to this track, Pragya (or Nikita as they say) knows a lot about him. She cares about him. She is understanding. Though he would have seen many crazy fans, she was still unique to him. She could identify his things even when blindfolded. She knows about his secret favourite coffee. When everybody including his Daadi believed that he misbehaved with Tanu, she said she wouldn't doubt him (she knows about him but to him, it must have been a big thing for her to believe him). He may not know about Aliya's evilness now but he certainly doesn't have the freedom to talk to her whatever he wants. That's why he shared his disappointment/fear about his marriage being rushed and his feeling about people hiding things from him to Pragya rather than any of his family members. She makes him feel comfortable to talk whatever he feels. She doesn't misunderstand him and sometimes that makes him not be uneasy to talk like 'he can't live without her', 'he can even give his surname to her baby', 'he would even take 7 marriage vows', etc. She makes him think aloud when he's with her.
He must have been longing for this kind of attention/love from childhood (since he lost his parents at an young age) so much that when he gets it, he becomes a child.
While it's debatable to say, its right to like/love a person just because they like/love you, you can't ignore the person when they care/love you so much.