In the chapter The Kingbreaker of A Dance with Dragons Barristan Selmy thinks:
All three of the sons of the fifth Aegon had wed for love, in defiance of their father’s wishes.
While I can find evidence that the two oldest sons Duncan and Jaehaerys II did it seems that Daeron never did. Then why did Selmy think this?
From a Wiki of Ice and Fire, sourced to The World of Ice and Fire:
When both were nine years old, Prince Daeron was betrothed to Lady Olenna Redwyne in 237 AC, a betrothal arranged by Daeron's mother. After his three older siblings, Duncan, Jaehaerys and Shaera had broken their own betrothals - Duncan to marry Jenny of Oldstones, Jaehaerys and Shaera to marry each other - Daeron broke his betrothal to Olenna as well. He did so when he was eighteen years old, in 246 AC. But while his brothers had broken their betrothals for another woman, this was not the case for Daeron.
While Daeron would not live for much longer, he would remain unwed.
Duncan and Jaehaerys did marry for love - Duncan breaking his betrothal to a Baratheon girl in order to marry the "strange, lovely, and mysterious" peasant woman Jenny of Oldstones, and Jaehaerys following directly in his older brother's footsteps, breaking his own betrothal to Celia Tully in order to marry his sister Shaera.
I don't know the context of the quote you mention, but what Barristan Selmy should have thought was:
All three of the sons of the fifth Aegon had failed to fulfil their betrothal agreements, in defiance of their father’s wishes.
Instead Selmy tarred Daeron (incorrectly) with the same brush as the older two, who did so in order to wed different women for the sake of love. Daeron did not; in fact, as Olenna Redwyne herself tells Sansa in A Storm of Swords, chapter 6, it was she who ended the betrothal and not Daeron.
