From your late twenties onwards, it becomes an annual event.
An old friend, family member or a guy you play 5-a-side with on Thursdays decides to get hitched, and the invite arrives.
Your first reaction is the be flooded with a sense of good will for the happy couple and gratitude you've been invited to the most important day of their life.
Your second? How much is this going to cost me.
The answer, on average, is £644.
Home insurer Policy Expert have ran the sums and concluded that the cost of being a wedding guest, including accommodation, gifts and the stag or hen do is the price of a week away on your own or a very nice watch indeed.
The lesson here, clearly, is to budget the best part of grand the next time someone you're close to pops up on Facebook with 'she said yes!' and picture of themselves smiling / crying on a mountain top.
That, or avoid having friend all together.