Things are impermanent: the poem mentions the opening and withering of flowers, which symbolizes the change and impermanence in life. He may want to remind you that it is normal to encounter highs and lows in feelings.
Fate comes and goes: the poem mentions that fate is like water, which means that fate comes and goes unpredictably, and we need to cherish the present. He may want you to understand that feelings need the joint efforts of both sides to maintain.
Release the sadness of the past: the poem describes the withering, sadness and tears of flowers, symbolizing the sadness and loss in life. He may want you to let go of your past unhappiness and focus on the present and the future.
Cherish each other: the last two sentences of the poem mention "for whom flowers bloom and fall, thank you, and for whom flowers bloom and fall", which means cherish each other and don't let good feelings disappear with the passage of time.
On the whole, he may hope that you can put aside your past unhappiness, cherish each other and work together to maintain this relationship. You can try to communicate frankly and resolve differences so as to better understand each other's ideas and needs.