The decision on a manuscript is usually based on more than one review, often two. As an editor you are faced with many different view from reviewers. Often reviews point in the same direction and may be differ by one providing a major revision and one a minor. But, it is no unheard of that one reviewer can suggest rejection while the other a direct accept.
In all cases, the editor should evaluate the reviews and arrive at some well-balanced decision for how the author should use the reviews to improve the manuscript (in addition, cases where MS are accepted without revisions are very, very rare; direct rejections are of course not so uncommon). If reviewers arrive at widely different recommendations and there are no obvious ways in which the editor can see how to reconcile the differing opinions, requesting an additional review by a third reviewer should be the solution. It is in this process where your question can appear because the way in which an editor values reviews depends on the scientific content, for one, but also on considerations such as the integrity of the journal and the publisher. Often such considerations lead to rejections rather than acceptance.
Accepting papers that have received more negative reviews can usually occur because the editor believes, for example, that the revisions suggested are not as severe as the reviewer indicated by suggesting (in your case) rejection, or, that the reviewer is off in the judgement as a whole. I have for example, seen rejection suggestions that are based more on antagonism than scientific reasons.
In the end reviewers make suggestions, editors (hopefully) provides an educated and insightful "verdict" that you need to adhere to in your revisions.
ad endum: When you receive more "extreme" negative reviews, you should ask yourself: is this because I am not very clear in some way? The answer is often yes!