Sometimes we put things off because the timing just isn't right; perhaps more often, we procrastinate because of a whole lot of negative emotions tied to a task overwhelm us the instant we think about a task we've already put off.
Behaviour blog Psychology Today discusses where these powerful feelings come from:
There's an emotional experience that I hear about (and have personally experienced) in relation to procrastination. It is the truly overwhelming feeling of negative emotions that we can experience just as we begin a "dreaded" task. I say negative emotions in the general sense, because there is such a mixture of emotions in the turbulent wave of emotions that people describe — fear, anxiety, frustration, guilt, shame, anger. One thing is common across these emotional experiences[:]we want to get away from these emotions as quickly as possible.
In the face of this wave of negative emotions, it's common for us to focus on short-term mood repair in favour of longer-term goal pursuit. Certainly we know from research and clinical work that one common, but self-defeating, way to deal with these negative emotions is to get away from the stimulus that seems to be causing them. In the case of procrastination, it's putting off the task at hand, because it's obvious that the feelings are precipitated by the task.
Once you've identified that wave of emotions for what it is, you're one step closer to tackling your procrastinated task. Or at least that's the hope. Really, though, who doesn't feel way better after crossing off a long-procrastinated to-do? It's just a matter of getting up the momentum to tackle it head on. Photo by Wevie Stilson (en Camaret).
Procrastination: Feeling overwhelmed, helpless and ready to run away [Psychology Today]