There could be a few reasons but I am sure you can figure those out yourself. I feel like we put off getting married for way to long but it was me doing it. When we decided to we just picked a day, scheduled time at the court house and took our families out to lunch after. We did a "reception" later in the year when there would be more time to plan and to invite people.

The only part that I say "wait a little while" is to take stock of experiences you have had already.

1. Have you traveled somewhere (ideally far away where neither of you have ever been). This can put stress on the relationship in a way that shows both of you how you deal with the stress.
2. Do you get along with each others family? You are marrying Her and the family (same the other way too).
3. Do you have separate AND common interests? You both should have your own hobbies/interests but they should overlap in places you both consider important. Like, do you both really like camping? That is a good overlap.
4. Do you enjoy each others company both talking and in silence? Looks fade but engaging conversation's do not need to assuming they have a good head on their shoulders and also #3.

These are all things I have suggested to friends that were trying to decide if they wanted to get married. The ones that listened are still married, the one that did not has been divorced for ~3 years now.

If you are so aligned have a conversation about this with her and ask her that exact question, general rule though, a proposal prior to 6 months is psychotic. I'd say a proposal anytime after a year of dating isn't too soon unless you are both under 20.