Iron atoms normally live their lives in complete chaos - spinning every which way like undergrads during finals week. But introduce a magnetic field? Suddenly they're lined up perfectly like freshmen at their first lab safety briefing.
This is ferromagnetism in a nutshell - those unpaired electrons in iron's d-orbitals get bullied by magnetic fields into aligning their spins. It's basically peer pressure at the atomic level. Nature's way of saying "get your act together or else."