The message: "Don't believe everything you think." Most people find that message amusing, for different reasons. Disturbingly many haven't and won't give it enough thought to be amused.
I like it because it italicizes everything, thus implying - like a good Jamesian pragmatist - that it might be okay to believe some things without compelling supportive evidence. Things that are "good" for life, that make a person healthy and happy and helpful and kind, ought at least to be entertained as candidates for our believing assent. That doesn't make them "true" in a traditional correspondence sense, but it might make them good enough. I'd prefer not to call them "true" just because they're useful, but I'd also prefer not to dismiss them out of hand as irrelevant to the larger purposes of philosophy and the total human enterprise.
I also like it because it's a necessary counterweight to the walker's impulse to blur the distinction between thought, belief, fancy, whim, etc. That impulse is perhaps best understood as free-form spontaneity, which is one of a good walk's great gifts. But it's a gift with entangling strings, strings that must be separated and sorted by the analyzing critical intellect. Speaking just for myself, perhaps, deliberate and careful critical analysis is not usually conducive to happy wallking. I believe I need to think some more about that.