Maybe you're not a morning person. Maybe you're deep in thought about what's ahead and not attuned to your surroundings. Maybe it wasn't something that was important to do around your house when you were growing up. But greeting people when you first see them each day is a show of respect. It says to an individual that you "see" them--meaning you acknowledge their presence, that they exist, and that they are important enough to address.
This issue comes up regularly when I am conducting internal customer service and self-awareness training sessions. For many, saying hello to a colleague first thing in the morning isn't a big thing. But for others, it's everything. Those who believe it matters are usually offended by the dismissive response they get from a coworker who walks through the office and passes their office or desk and never looks in their direction. They are particularly irritated when they offer a greeting first and the other person doesn't respond. They feel overlooked, minimized, neglected. It's very much psychological. It's about how we think others see us. And if we think they see us as insignificant, we feel disrespected. Especially if we have a high degree of respect for ourselves.
For those who don't care whether they get a greeting or not, they think the other side is making much ado about nothing. They weren't raised to walk into a room and say "good morning" or "good afternoon" so it doesn't mean as much to them. They tell me that they aren't trying to be disrespectful and don't see the lack of the gesture as being rude. It's not personal with them. They will make the effort when it's brought to their attention, but in the grand scheme of their day, saying hello when they walk through the door isn't a priority. Getting coffee is!
In an office setting, however, the act of not "speaking" to a person when they are first encountered can damage professional relationships. Coworkers want to feel that they matter. To be disregarded in something as simple as a greeting indicates to the person being ignored that their existence is unimportant--so unimportant that they don't deserve the smallest of acknowledgements. In the south where good manners determine how you were raised, the negligence of a greeting carries even greater weight and may even call into question the quality of your upbringing.
When I've seen heated exchanges among team members at work about this topic, I've often found that the behavior is split down cultural and regional lines. It's not just how a person was raised but also where they were raised that distinguishes the opposing views. According to the participants, greetings are usually a behavior that starts from early rearing. It becomes a requirement (or not) in the home, so by the time a person reaches adulthood, it is a common practice. For those who don't follow this practice, they say it wasn't something emphasized in their home. For those who do follow the practice, it is a hard habit to break. Not that they want to. They believe acknowledging others is polite, shows kindness, and warms up interactions as much as a smile does.
As a matter of resolution, I usually advise those who don't regularly offer greetings (especially managers and supervisors) to weigh the importance of doing it versus not doing it. It only takes two seconds or less--literally. Good morning. Good afternoon. Hello. A nod of the head and a smile. If it matters to the other side but not to you, then what is it costing you? Put the needs of your coworkers first. Do what builds better relationships. The antithesis, however, does bring a cost and it looks like this: an undercurrent of resentment between managers and direct reports, a wedge between coworkers because the greeting that could conjoin them doesn't exist, and a loss of respect for the offending party. Remember, respect is earned, not always automatically given. But automatically giving a greeting can certainly earn respect. Try it.