The short answer
Feeling behind in life is usually the result of comparing your own timeline against an internalized, largely unconscious cultural timetable for when things like a career, a relationship, or a home are "supposed" to happen. Psychologist Bernice Neugarten called this the social clock — a shared, culturally specific sense of the right age for major life events — and her research found something specific: it's not the milestones themselves that predict distress, it's whether you experience them "on time" or "off time" relative to that internalized schedule.
The research
In foundational research on age norms, the large majority of respondents agreed within a narrow age range on the 'ideal' age for milestones like marriage, parenthood, and retirement — and people who experienced these events 'off-time' relative to that shared timetable reported more psychological distress than those who experienced them on-time, regardless of how positive the event itself was.
— Neugarten, B.L., American Journal of Psychiatry, 1979
Why the feeling is so consistent across people
Neugarten's original research found a striking level of agreement on these timetables — the large majority of people in her study landed within a narrow age range when asked the "ideal" age for marriage, parenthood, or retirement. That consistency is exactly why the feeling of being behind is so widely shared and so specific: you're not making up an arbitrary standard, you're bumping into a real, culturally shared one, even though it was never something you consciously agreed to.
Why on-time milestones don't feel like a crisis
One of the more useful findings from this research is that events happening "on time" — even objectively hard ones — don't tend to register as a crisis, because they've been anticipated and rehearsed in advance; you've had time to build coping resources around them. Off-time events, even good ones, lack that rehearsal, which is part of why being "behind schedule" can feel destabilizing in a way that's hard to explain to someone whose timeline matches the norm.
What actually helps
Because the distress is tied to the mismatch with an internalized timetable rather than to the actual state of your life, the most direct relief usually doesn't come from catching up to an arbitrary deadline — it comes from making the mismatch itself visible: noticing when the feeling shows up, what specifically triggered it, and how often it's actually occurring, rather than treating it as a permanent verdict on how your life is going.