Most of us have a sense of "self". The boundary of "us" is our body. At least that is the boundary we are most aware of. It is also a natural boundary because its parts rely on each other for their survival. Loosing a limb will definitely diminish the chances of survival for all the constituant parts. So the self acts more or less like a queen over her hive, her primary role is to make sure the part peacefully cooperate to serve one goal: the survival of the unit and ultimately the reproduction of the organism.
It may be interesting to know that our body really is a pact between its cells. It was a major step in evolution when single cell organisms "decided" to join forces into a multi-cell organism. For most animals these cells are stuck together, but this distinction does not seem necessary. How about bees in a hive or ants in a colony? Do we consider the colony or the ant as the natural unit of organism? Also, even inside our body not all is peace. Sperm-wars and strong competition between chromosomes have been reported in the literature.
Even with humans it is not clear whether the body is the natural unit to claim a self. Interesting experiments have been conducted where researchers have measured the electric conductance of the skin of human subjects. These subjects were confronted with realistic looking mutulations of parts of their body. For instance, by using mirrors one can stage a situation where it looks like your hand is about to be hit hard with a hammer. This "would be" mutulation elicitated a stress reaction of the body which could be measured. However, similar reactions can be measured when virtual harm was done to one's direct family members, especially ones with which you share genes such as your children. Bizarrely, males also showed a reaction when their car was damaged (no kidding.) Researchers concluded that people also have an extended sense of self, or an "extended body" so to speak, which includes family members.
Reversely, some stroke patients seem to reject parts of their own body as belonging to "them". And then there are of course the patients with multi personality syndrom, where multiple "selves" claim a single body. It an illness allright, but it shows that the mechanism can break down. What we think as utterly normal may simply be one possibility out of many.
Perhaps we have many senses of self with a varying magnitude of commitment. From body via family or community to country. We can ask ourselves why people are willing to fight and die for "their country". What is inherently good about the country you happended to get born in? Isn't it odd that for every soldier on one side of the border that strongly believes in the cause s/he is fighting for there is another one on the opposite side of the border? Perhaps we have drawn another somewhat arbitrary line in the sand and created a "country level self". Maybe we should be working on a global sense of self, encompassing all of humanity. If only we would experience the suffering of 80% of humanity in underdeveloped countries as our own suffering, then there would most definitely be much less of it.