When you are vain, you love yourself. It is nothing wrong with that so as long as you keep it normal, it is certainly not a sin, but the phrase »Vanity is my favorite sin« refers to whole new level of vanity where the person loves himself and puts so much effort on his appearance and outer look it begins to bother other people.
Vain person is making a sin because he is making a huge waste of time making himself look better and better while he ignores other more pressing issues. The person is also wasting a lot of money in order to satisfy his appearance and achievements. But why is it his favourite? Because he clearly enjoys it and he enjoys the attention he gets. From another point of view, vain person wants to be the center of attention at all times and he is enough self confident to achieve that and with it give the preference to friends, co-workers and people in general. Vain person wouldn’t even consider giving the opportunity to somebody else because he is sure he does everything better than any other man could.
Making vanity a favorite sin can be a positive thing too. Being sure that you are better than others and accomplishing goals and assignments to prove that you really are the best is no doubt a good thing for person’s career, business and self-esteem, but it can seriously affect your relationships and consequently you and your relatives are doomed to suffer.