Well in this case 'llover' (to rain) is used like a metaphor for the sad parts of life , when the clouds are grey and it starts raining it's not particularly happy. So with that on mind, this talks about happiness being the right way to feel when it rains or 'llueve sobre mojado' (it rains on wet ) , which means that it rains on something that is already wet from something else , it's not gonna change much. When something bad happens and then, after a while , something else happens -> 'llueve sobre mojado' . 'siempre que nos dejen' means 'as long as we are allowed' . So be happy when something bad happens, even if it is one thing after another , as long as they (society,family,peers,etc.) let us (be happy). It's a very abstract and metaphorical sentence, so it takes some time to fully understand it. Saludos

"Llover sobre mojado" is an idiom that means that something has happened to you many times before so it doesn't affect you anymore. It can also mean that something is useless. So the sentence means that happines is the place to be always that it something bad happens even if it has already happened before always that they allow you to be there. I hope I have helped you, if you ahve any question don't hesitate in contacting me. Cristina.