This paper investigates the causes and welfare consequences of unravelling in two-sided matching markets. It shows that similarity of preferences is an important factor driving unravelling. In particular, under the ex-post stable mechanism (the mechanism that the literature focuses on), unravelling is more likely to occur when participants have more similar preferences. Moreover, any Pareto-optimal mechanism must prevent unravelling and the ex-post stable mechanism is Pareto-optimal if and only if it prevents unravelling.