I honestly believe there are very few people who are "afraid" of having someone who loves them through thick and thin. In fact I suspect everyone wants that eventually. A lot of this "fear of commitment" is a label used by lovers/mates of people who are upset because they do not see them as being "the one".
Another common expression is "He/She doesn't know what he or she wants". If a person is indecisive it's a "clue" they don't believe you are "the one".
The most important question then becomes, "Why would I want to commit to someone who isn't sure they want to commit to me?"
Your "soul-mate" will always be someone who recognizes you as being THEIR "soul-mate". Anytime you have to "sell", "persuade", or give an "ultimatum" in order to get a person to (commit) to you then you are with the wrong person. Every (serious relationship/marriage) should at least start off with BOTH people wanting the same thing.