Explain

love in friendship never ends

Presumably unlike the love in say a marriage, without friendship too, there's a big risk that it might not survive. True friendship, is probably the truest kind of love and also the most tolerant and long suffering.

Goddess brings up a good point. In friendship there is a whole set of intellectual and emotional connections and appreciation which is relatively uncompromised by lust and sex. In a love relationship you get all these things jumbled together and it tends to be more volitile.