Hmmm... Very true. I think this applies to more than judgement of others as well, and also applies to ways folks preach at others while genuinely thinking they are helping them.
I think in life we discover things about ourselves that are exciting to us; be they spiritual growth, or just growth as a person. We think we gain some level of knowledge we lacked before. We get passionate about this stuff.
Sometimes we see others who are struggling with things that are similar to what we've struggled with in the past, and we feel like we can help them in the issues they face now.
Therein lies the problem with trying to tell them the error of their ways, though. It's an illusion to think one has any kind of better information than someone else on what things they are dealing with, because their problems are subjective, as were yours. Any higher level of knowledge we've gained only applies to ourselves, and our own personal life experiences. No matter how similar their problem is to one you've experienced yourself previously, it's impossible for that person to tackle the problem the same exact way considering all of the other issues that would be attached to the problem that you may not know lie beneath the surface.
I've found that if people want one's advice on something, they'll ask; assuming they know about your experiences on the issue at hand. Chances are, if they do ask, it will pertain to a specific aspect of the problem they face. They have to live with the issues they face, so they are well aware of the beast they wrestle with.
Sorry if this is a bit of a word salad... I had a little too much fun last night and am still recovering my mental faculties.