Love here, is described as an “ever-fixed mark” which could be in reference to a star as well as being linked back to the idea that love doesn’t waiver. Also, the speaker is suggesting that love is not left open to storms (“tempests”), which again, is an extended metaphor to mean any obstacles that would taint love in a relationship, such as infidelity or lies. So to an extent, the speaker is comparing the way that a star guides travellers, to love guiding people, either to each other or towards where they’re supposed to be. Essentially, even if they don’t know where they’re going, love will take them