The short answer
Life-stage comparison tends to spike specifically during periods of transition and uncertainty — leaving school, changing careers, entering or exiting a relationship — because those are exactly the moments when you have the least internal sense of whether you're doing well, and other people's progress becomes the nearest available yardstick.
The research
Research on the modern adult life course has identified a normative developmental period — sometimes called the quarter-life crisis, typically occurring between ages 25 and 35 — that wasn't accounted for in earlier lifespan models, driven partly by how much more varied the timing of adult milestones (marriage, homeownership, career stability) has become compared to previous generations.
— Robinson, O.C., "Emerging Adulthood, Early Adulthood, and Quarter-Life Crisis: Updating Erikson for the Twenty-First Century", 2015
The transition connection
Research on the modern life course has identified something researchers now call the quarter-life crisis: a normative period, typically in the twenty-five to thirty-five range, marked by comparing your progress against peers and against an idealized sense of where you're "supposed" to be. This wasn't accounted for in older developmental models, partly because the social markers of adulthood — marriage, homeownership, a settled career — now happen on far more varied timelines than they used to, which removes a shared reference point that previous generations could at least assume.
Why it feels worse now than it might have to your parents
Part of what's changed isn't the comparing itself — people have always compared their lives to others' — it's the loss of a single, widely shared timeline to compare against. When adult milestones happened on a more standardized schedule, comparison had a narrower range of outcomes to measure against. Now that timeline has fragmented, comparison doesn't have fewer targets — it has more, and more varied, ones.
What tends to reduce it
Because this kind of comparison is heaviest specifically during unstructured transitions, the more durable fix usually isn't eliminating comparison altogether — it's building some internal sense of progress that doesn't depend on watching other people's timelines, which research on this period frames as one of the core developmental tasks of navigating it well.