If you wish to enter the mindset of the guest, it is crucial that you understand that the guest does not see you as an actual human being. This is the Second Noble Truth of Service. If you were to press the guest on the question of whether or not the person in front of them was a living, breathing individual - one with his own life and cares, tragedies and triumphs - they would of course concur but, truthfully, only after some consideration. The default attitude of the guest is simply to see you as a means of conveyance - a conduit of their food and drink from the ether to the table. Do not take this personally. It is simply the nature of things. Would you take it personally if a scorpion were to sting you? It is the nature of the scorpion to do so - just as it is the nature of the guest to have the self-centeredness and self-importance of a two-year-old.
To avoid killing your guests, envision them as already dead.
When we look at the world around us, very rarely do see things as they truly are. What we usually see is a conglomeration of images, assumptions and categorizations made by the mind. In other words, we don't usually see reality but rather our ideas about reality. For prehistoric man this was no doubt a helpful adaptive trait; it's much more useful to see lion = danger/run & hide, then to see the complex interplay of past and present, environment, ecosystem and co-dependent arising that is the actual "lion" (and I have to put "lion" in quotes because we're still just talking about an agreed-upon concept. In Reality, this thing called "lion" is actually its mother and its father, and the antelope it eats, and the water it drinks, and the earth, and the sun and the air and the entire universe itself. Again, not really useful when you're running for your life).
We do this every day, especially in the myriad of social interactions that make up life in the Industrialized World of the 21st century. When you check out at the grocery store, or get on the bus, or order a drink at the bar, your mind is constantly sorting and categorizing, putting everything and everyone you see into neat little organizable boxes. You might see Black Guy, or Gay Guy, or Hot Chick, or Zitface, or Bum, or even Slightly Surly Indie Coffeehouse Barista and Thinks He's So Cool Hipster DBag etc, etc. It's immensely rare that we look past our first impressions to the actual flesh and blood human being beneath (mainly because it'd be pretty hard to get anything done). So it should come as no surprise that guests do this all the time with their servers, and we do it all the time with them.
All this may make our social interactions slightly more efficient, but it has the added cost of fostering and supporting misunderstandings and prejudice. It's the dirty little secret of the service industry that a lot of us have some not so nice assumptions about our clientele based on things like race, and nationality, and class, and many servers do allow this to affect their service and their attitude. Of course, when a server approaches a table with a preconditioned hostility the guest is usually going to pick up on it - at least subconsciously - no matter how well the server thinks they are hiding it. Then the guest has some hostility in response - again, maybe just subconsciously - which the server picks up on, which only serves to "confirm" the original bias. I can't tell you how many times I've heard a server say "I can't believe I have to deal with these [fill in the blank]
What's the solution? Our brains just so naturally default to putting people in boxes. For my part, I really try to be mindful when approaching a table, to be aware of how my mind is instantly sizing them up and try to replace that with at least an understanding of them as an actual person. It's especially hard when a guest is rude, to not immediately think "rich pr*ck" or "stuck-up _____" etc, instead of looking through that to see someone who themselves may be hurting, or unhappy, or had a really bad day or maybe even a bad childhood. Maybe they are being a jerk right now, or self-centered, or demanding, but they also were once an innocent, smiling one-year old. At some point in their life they too have suffered tragedy and loss - they've had or will have a loved one die, and they themselves will face sickness and death. They have known fear, and sadness, and disappointment, as well as joy and love. That is not to say we allow ourselves to be doormats, or continue to put up with unacceptable behavior, but that we look through that to the flawed and fragile human being that lies beneath.