More questions about 「愛燦燦」。

ame sansan to kono mi ni ochite,
wazuka bakari no un no warusa o urandari shite,
hito ha, kanashii, kanashii mono desu ne!
soredemo, kakotachi ha yasashiku matsuge ni ikou!
jinsei tte, fushigi na mono desu ne.

kaze sanzan to kono mi ni arete,
omoidoori ni naranai yume o nakushitari shite,
hito ha kayowai, kayowai desu ne!
soredemo, miraitachi ha hitomachigao shite hohoemu!
jinsei tte, ureshii mono desu ne.

ai sansan to kono mi ni futte,
kokoro hisoka na ureshi namida o nagashitari shite,
hito ha, kawaii, kawaii mono desu ne.
ahhh, kakotachi ha yasashiku matsuge ni ikou!
jinsei tte, fushigi na mono desu ne.

soredemo, miraitachi ha hitomachigao shite hohoemu!
jinsei tte, ureshii mono desu ne.

So it seems like we have three main thoughts here:

1. When rain falls brilliantly on you, and you spend your time regretting recent misfortune, that's pretty sad; still, the past rests gently on your eyelids. Funny thing, life.
2. When you're buffeted by stormy winds, and you're crying over dreams that aren't working out, you're pretty weak/fragile. But in the future, someone you're waiting for will show up. Happy thing, life.
3. When love rains down on you, and you shed the secret happy tears of your heart, what a cute thing that is! But the past rests gently on eyelids; funny thing, life. And, still, in the future, someone you're waiting for will show up and make you smile. Happy thing, life.

Can't claim to understand that so very well. And there are lots of points of grammar and meaning that I'm not getting: Why "心ひそかな嬉し涙"? Why not で? Would the difference be "secret happy tears" vs "secret, happy tears"? And why isn't it 嬉しい涙? Are 嬉し涙 their own concept? And what are all these さんさん and さんざん doing? And why is かわいい an appropriate adjective in this song? And why are 未来達 and 過去達 plural? And what does it mean that they 易しく睫にいこう?