If someones first impression of you is a negative one because you are lazy or grumpy then they will look at you as a negative person and who may not get along with them to well. Also having a negative outlook at your job can be a bad thing for you when it comes time to get a raise or time to lay someone off.
You can be a productive worker but if you do not have a positive attitude you can be looked at as someone who is lazy and does not do much for the company.