Why do a lot of words begin with be?

The prefix "be-" is commonly used in the formation of verbs (think words like beheading or become, both are verbs). If you consider the English verb "to be" (conjugated to am, is, and are) in its un-conjugated form "be", this use of the prefix implies that something is existing or happening (a head is being chapped off, something is coming to be or exist). Also, the "be-" prefix as an un-conjugated "to be" verb can be used to imply location (such as when we say "I am here", "it is there" &c.). This implication is in use in the examples of behind (existing to the back of), before (existing to the front of), and beside (existing to the side of) that you provided in the question. Hope I helped! Sorry if this was a bit wordy, but so is etymology (pun VERY intended).