Why You Shouldn't Finish Their Sentence

Why finishing someone else's sentence does more harm than good. 

As someone who routinely does this...unfortunately...I know firsthand that this is a habit that comes off as rude and even elitist to the person you're speaking with.

We do this because we think faster than we speak. While someone is finishing their thought, we're tempted to jump in and save the day. The problem is that this makes the person feel inadequate, even though you have the clear advantage. 

I've done this many times during job interviews on both ends. When being interviewed, I felt like I was impressing the interviewer, when I was actually making him or her uncomfortable.  

Keep in mind that in order to finish someone's sentence, you have to interrupt them. We already know that interrupting others is a bad idea, so the same goes with trying to insert your own thoughts into someone else's sentence. 

Additionally, you may believe that by finishing someone's sentence you're telling them that you're following along. What you're actually telling them is that they need to hurry up and finish their sentence. Can you see why that can be annoying to others? 

An exception could be if they are clearly uncertain on a word that they want to say. In these cases, providing a few options is seen as helpful. If you want to let them know that you're following along with their thoughts, ask questions that pertain to what's been said. This is far more effective.