For the length of the answer, make it 100 words or less. Your interviewer does not have all the time in the world to hear you keep talking about yourself. As for the content of the answer, include 1-2 points about you as a person (maybe even something interesting to get them engaged) and about 3-4 points targeting the requirements of the job you have applied for.
The reason can vary for different groups of people. It highly depends on what kind of job you have applied for, whether you are applying for a part time job, an internship, entry level, managerial, change of industries, etc. Find a reason that suits you, your past experience and the role of the job you have applied for.
This can be a little tricky to answer. The way you phrase your answer can greatly impact your first impression to the interviewer. It may come across as too pushy and show off, but if done right, it can further secure your hold on the job. It is best to make use of your past experience to support your points. Experiences can be from leadership roles in school (can be applied to people relations, leading a team, prioritisation, time management, etc.). It can also be from difficulties faced in your previous job (can be linked to problem solver, prioritisation, leadership, responsibility, etc.).
Alike the previous question, this question can cost your entire interview. It is best to give some thought to this question before the interview. Also, saying nothing is definitely not an option!
Another way is to say that you are currently working on the issue to reduce the negative impact it may have on you as a potential employee of the company. This may even help in letting the interviewer view you as somebody who is self- reflective and able to accept change for the better.