I'm going to come from a different perspective than most posters. I'm looking at this from your SO's perspective. You went into his phone and deleted a text. Yes, it was your text to him, but it's still a boundary violation IMHO. You went on HIS phone and deleted something. In his place I would be upset. I can understand that could lead to him not wanting you to access his phone. It doesn't necessarily mean he is hiding something. It could be he is protecting his privacy. Many have mentioned you trusting him, perhaps he does not feel he can trust you to stay out of his phone?