I have a feeling that Sunburst never intended to leave Starlight like that. That he was just so caught up in the moment he never had a chance to say his goodbyes. And from what I'm guessing, his parents were in such a hurry to get him into the school they were talking about (I don't remember), he never had a chance. And if they do show him again, he's so caught up on his work and perhaps a high standing among ponies in Cantorlot, he can't make any time to rekindle his past friendships or for to his high standings, can't risk of being looked down upon and perhaps lose everything he worked for.