When analysing phosphoproteins via Western blot, why is total protein level of the target protein recommended as an internal loading control?
It is because you don't know how your treatments affect the expression of the total specific protein; perhaps it down- or up-regulates the amount of protein being made, but the relative proportion of phosphorylation doesn't change. For example:
Imagine you have 4 samples from different treatments of which you have loaded an equal amount of total protein (lets say 50 ug total, note that this is too much to load on a WB for accurate quantitation; usually you want less than 10 ug). You run a western blot and find the following numbers (completely made up BTW) for the specific protein. In this scenario, 1 is untreated control.

10 ug
20 ug
10 ug
20 ug

You then try the phospho-specific antibody and get the following

1 ug
2 ug
3 ug
1 ug

What does that tell us about the 4 treatments?
Well, it tells us that 2 is up-regulating the total protein, but the relative proportion of phosphorylation is the same. In 3 phosphorylated protein is up-regulated (3x) but not total protein and in 4 total protein is up-regulated but phosphorylated is down-regulated (0.5x).