You're only impaired if you're drunk. Impairment begins with the first drink. Tolerance is a good thing. Tolerance is a polite word for need.
Many factors affect tolerance.
Age, experience with alcohol, size, genetics, food in your stomach, attitudes, gender, emotional state, and an innumerable list of other variables all change how a person reacts to alcohol. It takes a glass of Kool-aid to have fun. See above answer about how tolerance is not cool. Also, alcohol creates a substance-dependent state of learning, meaning that if you learn to do something while impaired, you'll only be able to do that while impaired. Many people learn to socialize, dance, or cut loose while drinking, so alcohol is literally the only way they know how to do those things.