Why would you say no to a marriage proposal after dating for a year?

Your SO pops the question on your one year anniversary. Why would you tell them no?
Updates
9 mo
Thank you everyone for answering so far. I chose a year as my friend had this happen but let’s say 5 years or 10.. the question wasn’t about how long the couple is dating. I was curious why would you say no?
Why would you say no to a marriage proposal after dating for a year?
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