So many of us humans tend to go through our days on autopilot, which is essentially the opposite of mindfulness. We act unconsciously or habitually, even forming thoughts and judgments without conscious awareness of what we are doing (or why or how well). We just react.
We spend most of our energy rehashing the past or rehearsing the future: wishing, hoping, planning, ruminating, missing, regretting. We are disconnected from what is happening in our lives — right now, in the present moment — and even within our own bodies and minds. In this mode, emotions seem to just sort of happen to us, and we might not acknowledge them, understand them, or realize we can control them.
Or we might try to dodge emotions or shut them out. Either way, this is a recipe for emotion to overwhelm us. When we are not in the moment, we don't actually feel our feelings, and that creates more of the very emotions we may wish to avoid. It also doesn't (and can't!) solve the problems we are trying to escape.