"It will never rain roses" means that the roses will never just magically fall down from the sky, as if it's raining. Roses must be planted in order to bloom so we can pick them. This is obviousy a metaphore for hard work, meaning that nothing comes easy, and success can never appear overnight, you must devote yourself in an effort to achieve success. And the ":" here is used to separate the sentence, because "when we want" connects to the line that follows, but for metrical reasons, because it is poetry, it has to be separated, but when it's read, you pronounce it in the same breath. There is no "." at the end of the first line because it connects to the next one. Hope this helps :)

Yes, the no period on the first sentence and the Capital letter of the second one confused me a lot. Thanks for your help!

As Dimitrije says so well, it is; "It will never rain roses: when we want to have more roses we must plant more trees." but as poetry the formatting stresses the rhythm and for some strange reason unknown to me capitalises the first letter of the line, even if it is not the first of a sentence.