This topic is a bit of an odd one. Because while in America, it just feels normal. We all are American since this is our homeland (not talking about foreigners visiting or illegals mind you). I feel American because I was born and raised here.
While in America, celebrating our country's holidays that helped found/protect our nation is very American.
Supporting gun rights is very American.
Abolishing slavery is very American.
Supporting others both domestically and internationally with aid is very American.
Fighting to help free countries that yearn for freedom and seek our help is very American.
(Fighting from physically being there to spreading information to advocating)
To me, being American is to support freedoms. To protect human rights. To help those in need. To live our lives as we see fit as long as it does not intrude on other's rights. To accept that not everyone agrees and life has many different views or values.
I see this frequently when around fellow patriots. There is an unspoken camaraderie as we all hold these values dear and will support others who hold the same. I think I felt proudest being among others who hold this dear. Respecting our Constitution, our founders, and the soldiers that helped build and protect our country!
"To love something is to try to make it as good as it can possibly be. The United States will always need patriots willing to work to improve. In this land of plenty, we're often distracted by easy entertainment and shopping aisles full of the latest fashions and gadgets. This country needs citizens who remember that America is about much more than the good life, patriots who are willing to strive for our best ideals." -The American Patriot's Almanac
This is what makes me feel most American: those who follow this ideal and work hard to improve our country to be its best while celebrating our past successes.