"Attitudes" are the opinions and feelings a person has about something. Opinions and feelings don't have to be based on logical or factual reasoning, so describing attitudes, which includes feelings, is harder than giving reasons. Attitudes can vary with every person since every person can have different feelings and experiences forming the foundation of their attitudes. But if you're equating attitudes with reasons, it is surely possible to identify positive and negative reasons a worker may have regarding joining a union.
One reason why a worker might want to join a union would be for protection. A union can provide support for members. There are times when management exerts pressure on workers. A union can help to provide protection from this aspect of managerial control. Another reason why workers might want to join a union could be for collective bargaining. Being able to collectively bargain contracts for work is a vital purposes of a union. Workers might want to join a union to be able to negotiate for compensation and benefits.
One reason why a worker might not want to join a union would be financial. Being a part of a union costs money. Workers might not want to see more of their pay diverted from their paychecks. Union dues can be quite significant over the course of a year. Another reason why workers might opt against going into a union would be to preserve their own voice. In a collective bargaining setting, union workers have to abide by a membership vote. For example, if 60% of a union votes for a policy, that means 40% must go along with it even though it is against their wishes. Some workers might not want to have their voice silenced in this manner. In order to be heard, workers might not wish to have to live with the "will of the stated majority."