This passage by Steinbeck alludes to the human tendency to make the correlation that, if a person's current situation was different, it would undoubtedly also be better. This common belief is sometimes identified with the words "the grass is always greener on the other side", colloquially speaking.
It also alludes to another tendency, which is the supposition that everybody else "has it better" than us. This is another wrong idea; just because we do not like or appreciate the things that we do have does not mean that everybody enjoys a much better or fuller life than us.
Steinbeck says that this tendency is often seen in disparagement. that is, in a derogatory way; everyone can agree that not appreciating what we have is considered to be a "bad" thing. However, Steinbeck also makes the exception to this rule by saying that it is precisely this penchant for acquisition, and for the betterment of our lives, what colors and delineates human existence.
In other words, it is not in the best interests of people to compare themselves to others and to wish for better and bigger things while disregarding what they already have. Yet, can that be that bad? What if we lived a life of pure contentment: we would never know what kinds of higher goals we could achieve; we would never do better for ourselves and our families. In fact, if we were to imitate animals and just accept whatever is "thrown" our way, we would be actually doing ourselves a disservice!
Therefore, Steinbeck treats our tendency to "want" as a double edged sword: while it is true that humans naturally would have inner wants and desires, we should be very careful and moderate about what we wish and want for.