I don’t know if it’s the same for you but what I’m wearing directly affects how I feel. If I have a meeting with a client, I dress very professional. I believe my outfit helps me get into the frame of mind that I need for the meeting.
On the flip-side, I firmly believe that people form an impression of you within the first few minutes of meeting you. I know I do. Call me shallow but the first two things I notice about a person are what s/he is wearing and his/her hygiene. If both of those are lacking, I strongly hope s/he has a great personality!