I think it's part tolerance, part set/setting. When you smoke regularly, it doesn't hit you as hard. I'm a med smoker and I haven't skipped more than 10 days total in about three years, so I don't expect great highs anymore. Sometimes I get them because I get high somewhere new. Sometimes I just get a new strain, which can make a significant difference. Sometimes I'm just in a better mood and the pot enhances that.
As for being high in public, I too am cursed with uber-red eyes. It makes me unnecessarily paranoid. There is some risk, as I don't live in a large enough town to be confident I won't run into anyone from work. My eyes have been a long-running battle for me to overcome mentally. Some days I just say "**** it", but most of the time I try to hide them.