There's a difference between 'reserved' and 'fake'. When you are greeting someone that you don't know well, it is customary to smile politely and greet them...but it's considered awkward to give an over-enthusiastic greeting to someone you don't know well. Such as...to someone you've just met in a store. So a polite nod and a smile with a hello is sufficient.
Once you know someone better and establish a friendly connection, you may notice that greetings seem much more genuine. For instance, the person you meet in the store is now known to you as a classmate you like or your neighbor. Now, the two people may smile at each other in greeting and stop and chat for a bit. They may give each other a handshake or a wave. Closer friends, confidants, neighbors and relatives might hug, or greet with other friendly demonstrations of affection like high fives, pats on the back, fist bumps or whatever suits their moods.