1) Asset returns are high in US, capital enters US from other countries, so $ appreciates

2) Interest rates are high and bonds are secure in US..so money enters US, $ appreciates

3) US is highly developed in terms of industries, services, higher education, skill, innovations and technology. So, capital enters US from other countries, so $ appreciates

4) $ is the international reserve currency. All countries fix their reserves and currencies with respect to the $.So, capital enters US from other countries, so $ appreciates