I'll own up to it, I did once. I had a strong suspicion that the guy I'd been dating for a couple of months hadn't really broken up with his last gf as he'd claimed, so once when he went out to the car to get something I peeked at his texts real quick and sure enough, he'd sent her "I love you" not ten minutes before he met up with me. I didn't flip out about it or anything because I wasn't really all that invested in things...I calmly admitted what I'd done when he got back, apologized for snooping, and said I figured we should go our separate ways.
That said, that's the only time I did it, and I was 19 at the time. I've grown up a bit since then and have a strong respect for other people's privacy.