MLB players put tremendous pressure on themselves and get it from their fans and team executives. Entering 2013, these guys feel the need to excel.
Expectations and market size obviously affect them. Pressure is often proportional to the number of people viewing a player or covering him internationally.
Professionals are motivated by money and/or championships. Individuals in line for their next contracts know what's at stake, as do those who don't have suitable backups behind them on the depth chart.
Even from upper-deck seats, you will see the following major league players sweating on game day.