Put simply, it means that things don't always go smoothly in "true love." Sometimes people think that love will make all bad things go away, that it will overcome all foibles, disagreements, whatever. Shakespeare is letting us know that this is not the case. However, in Sonnet 116, he makes the equally important point: "Love is not love which alters when it alterations finds." Despite the changes and the hard times, true love will find it in itself to overcome and endure.