What makes a person drink and get behind the wheel ...

Arrogance.  Most people are convinced that they are flawless drivers, (they aren't) and most drinkers are convinced that they are in control, even when alcohol has obviously impaired their judgement & reflexes.

It's worse with young people, who haven't learned responsibility or faced their own mortality.

As a person who has lived with an alcoholic husband who would drink and drive I would say it's because they are so stupified from the alcohols addictive substances. It takes control over them not the other way around. I also have two daugthers who are adult children of an alcoholic and they both drink and drive. One of them paid a high fine and had her license suspended for 6 months. I had to drive her back and forth to work because of it. She'll admit she is an alcoholic but also says she can stop when ever she wanted to. She won't admit to the hold it has over her. And even after all that she went out and drove while under the influence again but didn't get caught by anyone but me! I was very upset to say the least. My other daughter is totally lost with her alcoholic problems. She lost custody of her daughter and has to have supervised visits when they are together. She is just trying now to get unsupervised visits. They are spot checking her at the court house. So, far she has managed to fool the system by not drinking around the time she thinks they might call her in. They usually give her a couple of days notice so she can be clean by the time she goes in. But she is only fooling herself and I wouldn't allow her to get those unsupervised visits knowing what she is doing. So, there you go. My ex husband diserted me and our four children due to his alcoholism. My daughter has done the same thing. The alcohol does control their lives. They do not know what they are doing. They just think that everything is all right when infact it isn't! I'm sick to death over my daughter's drinking. I wish I knew how to stop them from getting behind the wheel of a car while intoxicated! It's an illness and they have to want to get help to stop but most of them don't. Yet, there have been some that have beat the habit. I would like to see all alcohol made illegal but of course that wouldn't ever happen in a million years so in that case there will be more deaths due to drinking and driving.... 

Scientifically, the answer is the first effect of alcohol on the person drinking is to lower your normal inhibitions.  That means you FEEL like doing things you wouldn't otherwise do in your right mind -- such as, further drinking beyond what you know is good for you; and possibly driving, which you know you should not attempt -- but your normal inhibitions are blitzed.  That's why you wake up the next morning wondering where you slept and who with.