ancient chinese always use ‘千’‘万’to describe a large numbers of sth. China is a country with a long history, so the usage of some words which used in ancient china has been continued. this character ‘万’in this phrase is used to describe the time I love is never end.

不知道我打中文你能看懂吗？当你对喜爱的人表达情意时，爱你一千年，爱你一万年，爱你一亿年等等这些都可以，并不是必须得说爱你一万年，不过最好别小于1000，这样就不能代表你的爱海枯石烂了。只不过在歌词，一些经典的电影对白里，需要有押韵的句子和词语出现，爱你一万年押韵感比较强，所以就被人广为传诵，这样一来，大家都习惯用这样一句话了。如果你有时间的话，可以听听刘德华的《爱你一万年》。《大话西游》中爱你一万年的表白： 曾经有一分真挚的爱情放在我面前，我没有珍惜，等我失去的时候我才后悔莫及，人世间最痛苦的事莫过于此。你的剑在我的咽喉上割下去吧！不用再犹豫了！如果上天能够给我一个再来一次的机会，我会对那个女孩子说三个字：我爱你。如果非要在这份爱上加上一个期限，我希望是——一万年！

In Chinese, 万 is the biggest quantitative unit（个十百千万）, so people easily using it to describe the most much things. As to "爱你一万年", it starts from a song of Andy Lau, and it's famous only in these several years.

lol it is actually a famous song by a famous HongKong singer.it once was a hit and then became a classic. 10,000 years dont mean precisely that many years. here it means forever.

In the sentence, we don't need to use 10,000(一万, ten thousand) years. We can also use 1000(一千, one thousand) years, or 100,000,000(一亿, one hundred million) years. All of these words stand for "forever" in such a sentence, instead of the actual period of time, because nobody can live for such periods.