i think i kinda understand why they break up although they still love each other very much.
i have experience this for two times in my life. but i am not sure if their situation are same as mine.
i had beeen wit my first ex for almost two years. we love each other but at the end we break up. why? it is because of the guy cant take responsibility to marry me in the future. and he cant promise our future together. so in the end we break up.
Second ex, we been together for 9 months. still the same reason. and break up.

the only advice is choose carefully before u fall in love to someone, understand them more first. coz most of the ppl is covered by a mask... take care...