First the meaning sentence by sentence: 爱是一种相互作用 - Love is such an act of interacting 如果只剩一方在爱，会很累 - It'll be a worrisome pang if one carries the torch while another doesn't at all 一切顺其自然吧。一切顺其自然吧 - So just let nature take its course So these words contain the core that a ture love depends on the lover duo who indeed love each other. Otherwise, all in vain. And as it's such loving affairs, i prefer the meaning of "顺其自然" is "let nature take its course" rather than "let it be" coz the latter has the comprehensive meaning and the former merely around the loving, right? Hope i could help u

那意味着不要再执着不放了，有缘千里来相会，无缘对面不相逢

it means that let it to be what it may be,you accept any result it will bring to you . when you say this idiom,it indicate that you have quit being entangled with something.

well, as others say, it just means let the nature take its course. the whole sentence may mean : love's the stuff which involves 2 people. if one of them doesn't love the other one, it will be tiring. well, let the nature take its course. actuallly, this sentence implys some pessimistic view that this love will end.

顺其自然 means let it be.