Why do you get a hangover with lager and not real ale or Guinness?

Your question is based on a supposition that is not proven. A person can get a hangover from any alcoholic beverage. Despite many urban legends to the contrary, hangovers are caused by your body's reaction to alcohol, which is a poison, in too great a quantity.

There is some belief/evidence that certain drinks may be more likely to cause a hangover than others, and this is related to the organic byproducts in those drinks. Generally speaking, it seems that clear liquors like vodka provoke less hangover than wine, whiskey, or beer.

The higher the alcohol content of your drinks, the more risk there is of hangover. Also, some people are much more prone to hangover symptoms than others. Drinking more water with your drinks also helps avoid hangover as a major cause is dehydration.

But you can get a hangover from any of these, including real ale or guinness. Sounds like someone is spreading an old wives tale here.