Why do lorries try to overtake other vehicles that are only going marginally slower than they are?

It takes a long time for a H.G.V. to get it's speed up if heavily laden , so rather than slow down and wait behind a slower moving H.G.V. the driver usually goes for it and overtakes but yes to gain speed when loaded with 40 ton is slow but needs to be done or we would have lane 1 packed with H.G.V.s and we don't want a line of H.G.V. s