You are confident in your looks, but you are curious or even find it a little suspicious that he/she is with you after a line of extremely gorgeous dates or relationships.
Here's what we have discovered.
1. Law of diminishing returns
Physical beauty is not immune to diminishing returns and at a certain point has low value in overall attractiveness.
To the outsider, an extremely gorgeous will always be that way. But to the person dating said gorgeous person, at a certain point, that person becomes "average-looking".
That is when other qualities and factors, especially personality, become increasingly important. Without these other factors, looking nice loses its value in dating.
2. Maturity
This does not mean that maturity means losing taste in extremely good-looking people. In fact, it takes a certain level of maturity to date physically gifted people.
However, his/her focus is on your character, personality and other qualities. Your partner has likely learnt valuable lessons in relationship compatibility, their strengths, weaknesses, they have outgrown bad dating standards.
He/she might have held those exes to impossible standards or himself. Plus, you are focusing on the looks, not them.
3. Avoid comparison
Never feel like you need to measure up to his/her exes.
Your partner is with you and not them. Consider it an upgrade if you find time to feel cocky.
First, you cannot get these people's looks, and if you want to compare, do it against yourself.
Some time down the road, you will change, what will happen then? Focus on being yourself, because that is who/what they fell for.
4. Talk about it
Only if you are brave enough and it is not born out of insecurity.
If you have that kind of bond with him/her where you feel comfortable to even address it, you have a good thing.
Remember, he/she might not have an earth-shattering answer beyond "they were not good matches", because when you love someone they are beautiful unconditionally.
5. It is what it is
Believe it or not, people are comfortable with people in their own level or limits (even slightly lower or higher). Social status, economic class, physical attractiveness, intelligence all exist and influence our search for friendship, relationships and romance.
Whether this happens consciously or outside our knowledge doesn't matter, it still happens. What matters is that we find our peers and we are happy.
Think about this: do you choose your friends based on looks? Do you even choose them? Half of the time you "automatically" choose each other.
Takeaway
The question of "why is he/she with me" is quite normal and innocent. Most of the time it is curiosity. But if it has insecurity attached to it, consider addressing it immediately.