Why do people think that Orthodox Jews are more serious than Reform Jews?

Answer 1

The word "serious" is odd. A person can be serious or silly without regard to degree of religiosity. As for whether Orthodox Jews follow Judaism more stringently than Reform Jews, this is simply a fact. Orthodox Jews are more concerned about following Jewish Law, more concerned about Jewish Theology, and more concerned about the Divine, than Reform Jews are.

Answer 2

Because Orthodox Jews are seen as being more principled, more loyal (to tradition), more contemplative and circumspect, and less likely to be led astray. This is because of their adherence to the Torah and its laws, values and attitudes.

Here is a small sampling of Jewish attitudes.

1) Every human being is as important as the entire universe (Mishna, Sanhedrin 4:5).
2) Our actions, thoughts and speech create ripples that spread throughout eternity.
3) Everything is a test.
4) The only temporal thing that you really own is time; and not much of that either. Every moment is golden.
5) To be Jewish, almost by definition, is to be happy, content, and optimistic.
6) Our soul (yes, humans possess a soul) - yearns for God; and this emotion won't be assuaged with food, money or pleasures.
7) The truth is much more interesting than fiction. The Torah will not only reveal its own secrets to those who study it; it will also reveal the secrets of history, and the inner workings of men's minds.