So your glass is half empty, isn't it? It sounds to me like you aren't fully enjoying the return greetings that you do recieve because you're too busy focusing on the ones that you don't. If that's the case, you might want to consider lightening up a little.

In the interest of full disclosure, I'd like to think that I acknowlege everyone I pass when I'm riding but it really isn't true. Sometimes I'm so lost in my own thoughts that I just return a barely perceptable nod or maybe even nothing at all. One of the things that I like best about bicycling is the freedom that it allows me to lose myself that way.