相思想愛 is a distinctive, poetic variation on the familiar phrase 相思相愛, meaning that two people love each other equally. In this song, the altered wording highlights not only mutual love, but also sharing the same thoughts and feelings, making it a memorable expression for the couple’s uniquely connected relationship.
"相思想愛" is DXTEEN taking a love song and turning it into a vow, and they commit hard. The title plays on the idiom 相思相愛 (two people in love with each other), but swaps the middle character for 想, "to think of someone", so it reads like thinking of each other, loving each other. The chorus opens with "一生涯 相思想愛" (a whole lifetime of mutual love) and promises "この恋は終わんない" (this love won't end). That 終わんない is casual speech for 終わらない, the kind of contraction you hear in real conversation, so it is worth learning.
What I like is that the song is not all fireworks. There are small, real moments: two people texting "'会いたい'" (I want to see you) at the same time, or the singer realising that just hearing "君がふと語る未来に僕がいる" (I'm in the future you casually talk about) is enough to give him a dream. He also admits he gets jealous: "君の話は全部 聞きたい" (I want to hear everything you say) but "でも過去の恋は聞きたくない" (but I don't want to hear about your past loves). Honest, and a little needy, which makes it cute.
Keep an eye on "いつもぶっつけ本番" (always doing it live, with no rehearsal). ぶっつけ本番 is an idiom for going straight into the real performance without practice, and here it means they are figuring out love as they go. He also wants "'ふたりなら'" (what works for the two of us) rather than "'普通は'" (what's normal). The ending gets sweet with "きらり ふたり いま両想い" (sparkle, the two of us, in love with each other now), where きらり is the sound of a twinkle and 両想い is the everyday word for mutual love. Is promising "春夏秋冬 100年後も" (spring, summer, fall, winter, even 100 years from now) a bit much? Absolutely. That is the fun of it.