Actually I was also surprised to see Sammy happy so quickly. Its like, in a flash of a moment he was sad n then became happy just coz Dia said dat he shud b happy. Till yet I was confused but I think its bcoz,
Life mein pehli baar Sammy ko sachcha pyaar hua hai. Every1 knows him as d flirty guy of his clg, even d princi aur har larki uske liye Flavour of the Month hoti hai. Ye pehli baar hua hai k koi larki uski zindagi mein 30 days se ziyada rahi hai aur us larki ne Sammy ko change bhi kiya hai. At Sheena's time, Sammy actually encountered d prb of Infatuation, u know Temporary likeness. Sammy ne na hi use propose kiya aur na hi use kuch announce karne ki zaroorat pari. Gunjan k muamlay (matter, issue) mein aisa hua. Till yet d whole clg knew dat they're best friends. Those who thought abt being them a couple, got rejected abt it by Sammy n Gunji themselves, remember last time. so actually nobody ever imagined dat he can fall 4 a girl like Gunjan although there were rumors.
When Sammy was sad, it ws coz he thought he lost Gunjan n wasn't able 2 tell hr his feelings bt when Dia tld him dat he shud b happy, its coz 4 d 1st time he's truly in luv. Dia just made him accept his luv in front of every1 widout any regret. It meant dat he shud b feeling happy, even though he doesn't know abt Gunjan's feelings bt he ws happy coz of his true luv which Dia made him realize at dat moment.
I hope I cleared ur confusion n ys, definitely mine also. I think this is d reason why he became happy in a sec after Dia said dat.
Zaro socho k agar kisi Sam jaisey flirt ko sachcha pyaar ho jaye toh woh kaisa feel karega. I think just Sammy.