So how do they know? How do they do it, this showing up when most needed thing? It is a grand mystery but here is my take. The field around us, the matrix of vibration we all swim in is like a giant message board. Every thought and feeling we have is up there somewhere. Dolphins are like the interpreters of some kind of Morse code, comprehending what is vibrating in the field, whereas to most of us, we just hear blips and bleeps of sound, having no idea what it means..
Dolphins are fond of humans and have been for eons. Why, nobody knows, but they are. Oh there is speculation a plenty, and I could make a pretty good case for why I think so, but the point is this. They can read heart emanations. When they pick up a signal coming from someone's sad or scared heart, they tend to go to the source of those signals. They may not turn it into a personal encounter. There may be no eye contact, but they are just there. A dolphin fin showing up (whale as well) is, I would venture to say, a universal sign for reassurance. Dolphins are the great fear dissipators on the planet.
When Burt Bacharach wrote the words "What the world needs now, is love sweet love, it's the only thing there is just too little of", he must have been channeling his inner dolphin. Dispelling fear is the way home and when fear leaves, love steps through the door. Dolphins are being very busy bees and certainly doing their part to help us humans feel loved, and safe. I think they are also saying "come on in, the water's fine" meaning, it is just fine for us to become like them and treat life as a great big fish banquet.