We place more weight on the present than on the future, and more weight on ourselves than on others. We may say that we worry about future generations, but our actions - how we invest our time and money, for example - indicate that we do not. Here is an argument for not worrying (from Jeffrey Krautkraemer):
If technological progress enables the future to be much better off than the present, then there [is] less need for greater weight being placed on the very long run.
Past generations did well in not worrying much about us.