1. It was about sex
You probaby thought that all the romantic things or dress up they did was about making it special for you too.
But most times someone is doing things like wearing lingerie, paying for an expensive evening, simply to feel good about themselves.
Depending on the time you were together, you were probably not well acquainted with each other's bodies to get intimate.
2. Unfulfilled expectations
First times, no matter how experienced you are, can be awkward with different people. Moreover, you are probably not comfortable enough to laugh about any awkwardness or mishaps.
It might also be hard to have the life-altering orgasm that both of you have imagined and flirted about in your chat.
There was nervous energy and tension you did not resolve afterward. You probably did not acknowledge the stress from the pressure of a first time.
If you neglect this nervous energy it can even made small mistakes appear bigger than they are.
4. It was too serious/performance
There was no sexual chemistry. It was almost mechanical and they could not wait for it to be over.
Anticipation can give the impression that you and a potential sexual partner will be a perfect match. On the contrary, other factors play a major role in the outcome. This includes communication.
Below the surface of first times is also a need to perform or impress rather than enjoy each other.
If the reactions and the whole experience does not match your imagination, it can bring embarassment or disappointment.
5. Good intentions
It was rushed. You both, or they, did it for the wrong reasons and expected a good outcome.
They felt pressured and caved in, wanted to prove something. The aftermath was regret, shame or guilt.