Where does “愛して” come from?

愛して (Japanese) comes from Japanese て, from Japanese つ, from Old Japanese つ.

Ancestry of “愛して”, step by step

愛して traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Japanese	て	A conjunctive particle, attaching to the...
2	Japanese	つ	indicates the completion of an action; indicates...
3	Old Japanese	つ	—

Step	Language	Word	Meaning
1	Japanese	愛し	dear, loving; loving
2	Japanese	憂し	—