shelbkr
Each person is connected to their soulmate by a thread only they can see. If you or your soulmate dies, a new string may appear after two months at the most. Those who do not have a soulmate have no strings, and are known to fall into a deep depression later in life.
On rare occasions, ones soulmate may change. The universe does make mistakes after all. Some find that after twenty years of marriage, their string no longer connects to their spouse.
And some find that after fifteen years of being unattached, a bright red string appears, connected to their heart, leading them towards a new friend. Or even, an old friend.
//some information
the strings are not tangible, aka you cannot touch them. They are a visual 'illusion' of sorts which can only be seen by those connected to each other. A new string is bright red, an old string may fade to a light pink colour.
A dying string, one that connects two people who are slowly drifting apart from the true love they shared, will start to become grey. Eventually it will turn white, and then disappear.
Phil Lester, normals to the naked eye, is unattached. He has never had a string. Never has Phil fallen in love, never been loved by anyone other than his parents.
Dan Howell is a drifter, an unattached who coerces people into relationships, even though they aren't soul mates.
Both have never truly been in love, but the universe does make mistakes some times.