Attachment theory, originally developed to describe infant-caregiver bonds, extends reasonably well to adult romantic relationships. Secure attachment involves comfort with both closeness and independence. Anxious attachment involves a strong fear of abandonment and a tendency to seek constant reassurance. Avoidant attachment involves discomfort with closeness and a tendency to withdraw when a relationship gets emotionally intense. These aren't fixed labels — attachment style can shift somewhat across relationships and with deliberate effort, but the underlying pattern tends to be sticky.
The most useful application isn't diagnosing yourself or a partner and stopping there — it's recognizing that an anxious partner's push for reassurance and an avoidant partner's pull toward distance often escalate each other in a feedback loop (the anxious partner pushes harder because the avoidant partner is pulling away, which makes the avoidant partner pull away further). Naming that dynamic explicitly is often more useful than either partner trying to fix themselves in isolation.
Hazan and Shaver ran their now-famous study through a local newspaper: a "love quiz" describing three short paragraphs — one for each attachment style — and asking readers which best matched how they approached romantic relationships. The self-selected results roughly matched the distribution decades of infant-attachment research had already found (about 56% secure, 19% anxious, 25% avoidant), and, more importantly, correlated with how people described their actual relationship histories: securely attached respondents reported longer, more trusting relationships; anxious respondents reported more jealousy and more emotional highs and lows; avoidant respondents reported a persistent fear of closeness.