To preface: I'm certainly not saying we shouldn't treat everyone with respect & kindness - that should always go without saying!
What I am saying is that the older we get the more we realize (or at least should) that not everyone is equal or on the same level. A white collar millionaire & a down home country girl will rarely make a good couple; their life experiences, political viewpoints, spending habits, hobbies etc are all (quite likely) so far apart that there will always be disagreements & unhappiness that will grow as the differences cause more problems. While this is an extreme example (& subject to exceptions) it applies to everyday life as well. As adults, our closest friends are usually the ones that have the most in common with us. They think as we do, have similar parenting views, similar education levels, similar professions - and thus they understand us so we feel a kin & comfortable. Interestingly people seem to be ashamed to admit this, they'll exalt that they're "friends with everybody" & "don't judge others" but yet they do; they compare their kids against others in grades & behavior charts, they talk about he said/she said, the virtues or viles of tv time/ electronics/ car seats & the like. It seems to me that as long as you are living by the cardinal rule "Treat others as you want to be treated" then it shouldn't be a point of shame to prefer certain company over other or admit there is 'X' trait you dislike.
Real life example 1:
We had a close friend Beth* who at the time I met her was working as a technician* with a high school diploma & few college courses, the other 3 of us had baccalaureate degrees. Viewed her as nothing more then a sweet, likeable woman with a wry sense of humor & similar interests. She was a good, dependable friend & likeable company but as soon as conversations turned anymore 'serious' then what's on tv tonight she'd either space out or do just about anything to steer the conversation back to her liking. As she set about getting her Associates degree, she'd complain how difficult college math was (to 3 engineers/scientists) and grew increasingly frustrated that we couldn't commiserate it with her difficulties. Never once did any of us belittle her abilities or talk about how easy her problems were really but we also couldn't lie so still she felt less then worthy due to differences beyond her control. This chasm only grew bigger as kids started to arrive until it erupted into a huge fight with her accusing us of "always making her feel stupid" & "lording our abilities over hers". You can be a as friendly & nice as you want but differences matter despite our intentions.
Real life example 2:
In VPK my daughter had a good friend named *Jane, after hearing about her for several weeks I finally had the opportunity to meet her mother *Kristy, both mother & daughter were a few of the sweetest people I've ever had the pleasure to meet. So both mothers encouraged the friendship between girls & also maintained friendly conversation between ourselves. I have a lot of admiration for this young lady because she's a single mom with a minimum wage job, even with so little she always strove to provide *Jane with everything even if that meant she had to do with less. One morning the teacher asked how I was doing on GRE review, Kristy overheard & asked if that was a variant on GED. I replied that it was for entrance to graduate school (a year later still a dream sigh!), she poorly hid a surprised expression & asked "so you already have a bachelors". After I relied that I did, had worked awhile after graduation before choosing to stay home with kids, she stopped having extended discussions with me then the basic pleasantries. Then at VPK graduation the teachers did a "who's who" of the kid detailing unique facts about them, Erin's was that she had completed every puzzle in the classroom & read on a first grade level. *Kristy told me the next day that she had changed her mind about play dates over the summer & wished not to be contacted again. Another case of friendship began with the best of intentions, 2 little 4 year old girls bonded over My Little Pony & a love of Rapunzel without a notice or care of any difference in their lives - the way we teach it should be but obviously don't live by (perhaps unconsciously but still). End result was a sad (now) 5 year old who couldn't understand why her friend went away - made even worse by the fact her zoning waiver was accepted & *Jane's wasn't :-(
I could go on with several more examples, recent & not, but that would be tedious to all I'm sure. My conclusion is thus; if we're honest with ourselves & admit that some differences do matter from the start while still adhering to the Golden Rule we'd save ourselves a lot of pain & guilt. Opposites may attract but birds of a feather flock together for a reason!