From what I've read about meth, it's a purely psychologically addictive drug with only minor physically addictive properties. That's because it's so different from the physically-addictive substances that make you feel goofy, tired, off-balance, and in general "fuck you up". Meth actually makes the user feel MORE sober and efficient with things like energy, confidence, and concentration, so it's very easy to convince oneself that you need it to function at 100% and very hard to go back to your old way of being, especially if you have a condition that makes you feel fatigued, depressed, insecure or unable to concentrate and work efficiently, even after only one usage.
Meth addiction doesn't make a person weak, stupid or "bad"; in fact I think it usually comes from a good place of genuinely wanting to feel good, relate well to others, and perform well in a world where people are overworked and underappreciated. The stresses of everyday life: rude co-workers, demanding spouses, children, bills, health problems, mental problems, and other stressors take a toll on a lot of us, leaving us devoid of energy and motivation...meth can restore that temporarily. Some of us want nothing more than to feel confident and excited about life like we were in childhood, and meth is a quick, easy solution.
That's not to make excuses for anyone who uses meth or other stimulants, and it doesn't justify using such harmful substances, but it helps to put yourself into another person's shoes before judging. I've never tried meth or any amphetamine, but can relate to a lot of users who cite insecurities, low self esteem, fatigue, stress, depression, and social anxiety as the reason they started using.