Because you never know who you are talking to. You want people to think the best of you. Say you are accused of pushing a kid at recess. You have never lied to your teacher (your teacher knows this) and so they ask you, "did you push Billy" You reply with "No" she is more likely to believe you because you were honest an left a good impression. Another reason why its good to have a good impression is if you are being interviewed for a job. You want the employer to like you and want to hire you, so you should always put your best foot forward.