Unlike existence, sentience is typically considered to be an emergent property; that is, an entity spontaneously becomes sentient because of its other properties.
For example, imagine three line segments that share no vertices on an otherwise empty plane. They aren't sides of a triangle, are they? Yet, if you connect their vertices, a slew of properties spontaneously emerge; specifically, the fact that they become sides of a triangle.
Although I'm unclear why we are using geometry to discuss biological/metaphysical phenomena, I understand what you are saying.
However, a triangle is no more than it's three lines. The fact that you and I are discussing our observations of the physical world and postulations about its origins shows that life is more than non-life.
Although you can derive angles, areas, and whatnot by combining three lines, you can't derive dynamism, choice, action, or awareness by combining a bunch of atoms. Something more than what the combination of the atoms occurs, namely, dynamism, choice, action, awareness - sentience.
A triangle is the sum of it's physical parts. We are more than the sum of our physical parts because we can do things, think, move, interact, choose, love, hate, and manipulate matter through our own will.