GRRM is a romantic at heart and he loves to play with tragic love and its consequences. There is a fragment from Barristan's POV that sums it all up in a few lines.
Prince Rhaegar loved his Lady Lyanna, and thousands
died for it.
The most recent example of what tragic love is, Rhaegar x Lyanna. We know very little about them but the last books have been hinting towads a love story, a tragic one but a love story nonetheless. And there is nothing more tragic than a couple that blossoms when a war is being prepared in Paris and Helen's style.
Daemon Blackfyre loved the first Daenerys, and rose in rebellion when denied
her.
This has been rumored to be one of the things that triggered the First Blackfyre Rebellion. Not the main reason but it played a part in it. Every character involved had different reasons and Daemon's seem to be a denied love, reciprocated or not.
Bittersteel and Bloodraven both loved Shiera Seastar, and the Seven Kingdoms bled.
Even if they were Half-Bracken and Half-Blackwood, this ancient enmity reached the Great Bastards and it was fueled by the love they both felt for their half-sister. The black dragons and the green ones were bound to fight but this love made the war(s) crueller than they'd have been in normal circumstancies.
The Prince of Dragonflies loved Jenny of Oldstones so much he cast aside a crown, and
Westeros paid the bride price in corpses.
A noble man and a commoner whose love crossed social boundaries but caused a lot of trouble. Specially after Duncan's brothers and sisters followed his example and married for love. If it weren't for Jenny and the ghost oh highheart, Rhaella and Aerys II wouldn't have married and we would be looking at a different story.
All three of the sons of the fifth Aegon had wed
for love, in defiance of their father’s wishes. And because that unlikely monarch had
himself followed his heart when he chose his queen, he allowed his sons to have their way,
making bitter enemies where he might have had fast friends. Treason and turmoil
followed, as night follows day, ending at Summerhall in sorcery, fire, and grief.
ADWD, The Kingbreaker (Barristan III).