Why should a scientist always share the results of his or her experiments?
Because science is a collaborative effort, and the good faith of your colleagues and coworkers is always assumed.
Science proceeds by experiment and observation. An experiment is done to observe the effects of a change in one variable; another experiment observes the effects of change in another variable. No individual or individual research group can do all the experiments; and the sharing of knowledge and experimental results allows another researcher to devote their time to new experiments.
Of course, new results are eventually published, but this may be up to 5 years AFTER the new experimental results are obtained. In the meantime, BEFORE the work is published, a researcher would think nothing of sharing his new results with other interested researchers. He or she assumes their good faith (and this good faith will be returned), and the other researchers may have insights or other experimental data that they will share with the first researcher.