And why does changing the angle of the glass change the amount of head?
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The head is caused by CO2 suspended in the beer coming out as bubbles. The foam itself is formed because beer has a much higher surface tension and so the bubbles do not pop as fast.
The reason changing the angle changes the amount of head is that agitation caused by pouring the beer into the glass causes CO2 to release from suspension. If you pour the beer down the side of the glass, you decrease the amount of agitation and less CO2 leaches out.
Beer foam forms because the gluten in the grains used to brew the beer traps CO2 diffused in the beer as it turns to bubbles when poured into a glass.
Carbon dioxide dissolved in the beer coming out of solution due to the decrease in pressure