Technology is a gift and a curse especially with the invention of Serato & Tracktor. DJ's tend to lose their momentum because they are focusing on their laptops and paying less attention to the people in front of them. It is imperative to keep an eye on the party so you can read the crowd. Watching their body language will help you decide what to play, or switch the direction you were trying to take the party to.
If you are too focused on your laptop, picking songs, and thinking whatever you play will get the crowd hype, you may miss the mark by a long shot if you don't notice that they are bored to tears, looking at their phones, and walking out of the venue. By the time you look up you will be the only one left in the venue playing music for yourself. Let's face it, if you are not catering to the crowd, who are you really playing music for?