Vanesa Martín opens this one with a kiss in close-up: "Con los ojos muy cerrados y la lengua ya dispuesta" (with my eyes shut tight and my tongue already willing). She is falling into someone, "tan segura entre tus brazos" (so sure of myself in your arms), and she is honest that she might not stay.
The chorus is the bittersweet hook: "el amor eterno dura solo lo que duran dos veranos" (eternal love only lasts as long as two summers). So the "endless summer" of the title is a bit of a joke on itself. Love is forever while it lasts, it burns, and then it fades "como un atardecer, cuando se va deprisa pero va a volver" (like a sunset, when it leaves quickly but will come back).
She also admits "No manejo el compromiso" (I don't handle commitment well) and "siempre me quiero ir" (I always want to leave). Learner tip: "me pilló por sorpresa" uses the very Spanish verb pillar, slang for "to catch", so it means "it caught me by surprise". And "tiro en blanco" is her twist on a shot in the dark: a free shot taken with nothing to lose.