Emotions are not totally a bad thing, but if one can be without emotions in trading and just focus on his unbiased analysis it will be more favorable to the trader.
Emotions can be a barrier to analysis. Sometimes your calculations may tell you something else, but your emotions will tell you something else. On a few occasions, your emotions might be right but it will just be fear.
Emotions like greed, and fear can make you make decisions that you ordinarily didn't want to make while trading and you can regret them later. So to me, it's best if you can just make decisions without any emotions involved.