Honestly, it's hard to look back and be objective. I've been in forums like this one since the start, and have come across some of my posts--I didn't recognize them being mine, and almost replied to them!

. I felt no connection at all!
There are many reasons people say they're going vegan, and not all are really in line with vegan ethics. The first thing I would say is understand that vegan does not imply health! If someone is interested in health benefits, that's plant based, which may be a subset of "vegan diet", but not necessarily vegan
As Lou points out, if you want to understand why people quit it really is better to hear from them. Hindsight is tainted.
I read far too many posts from those who've quit that say they couldn't get the proper nutrients, and felt like crap, but never fully disclose what or why.
Personally, I wish people would focus on understanding the change by incorporating more meals that don't include animal products rather than an abrupt switch. Meat dairy and eggs are the focus of the vast majority, and I do know it took me months before I felt it was my 'normal', and I didn't have to think so much