Personally, I feel what Sameer did was to an extent wrong.
I agree with you, trust is indeed a very important aspect in a relationship; it takes a long time for someone to have utmost faith in you and no time at all for it to break.
Sameer's intentions weren't wrong, he didn't want to burden Naina with all the stress and pressure, especially during exams. He knew how much academics mean to her and didn't want to hurt her by revealing the truth. Which is why when he 'pranked' her by actually telling the truth and hearing her flabbergasted reaction, Sameer grew more determined to never let her know.
However, he failed to realize that by hiding things from her, he's hurting her even more.
Naina trusts him, and thus she shares all her worries, all her pain with him. Whether it be family matters or problems in college. She too hopes for the same from Sameer. By knowing that he has been hiding things from her, Naina will feel that she couldn't establish that faith, that Sameer didn't trust her enough to shed all his inhibitions and fear and share everything with her.
But there are some things that we learn from experiences. Maybe Sameer will realize his doing, maybe Naina will help him overcome his fears.