Why can't the speaker enjoy the beauty of the woods?

The poem doesn't give readers an explicit and concrete answer to this question; however, the poem does offer enough evidence for a reader to hazard a guess or two as to why the man can't stay stopped by the woods. One possible reason as to why the man can't stay stopped is a straightforward and realistic reason. It's cold, he's getting cold, and he needs to get himself and his horse to shelter. We are told that the man stopped at night, and night temperatures are more often than not colder than day temperatures. Additionally, it is cold enough to snow. He's watching the woods fill with snow, and we are specifically told that the lake is frozen. In order to freeze a lake, it has to be quite cold for quite some time. We aren't given any indication as to how long the man has been riding or how much farther he has to go, but delaying any longer could potentially be life threatening.
The final lines of the poem give readers another option as to why the man can't stay staring at the woods. We are told that he has promises to keep. Maybe he promised his wife that he would be home by a certain time, and he had better not wait too long. We have no idea what those promises are, but we do know that he feels a strong need to get to his destination in order to keep those promises. He may want to stay looking at the "lovely, dark, and deep" woods, but his commitments to other people are more important than his own desires.