Definition of High Maintenance: "requiring a high level of care and attention".
More effort + work = stronger ties
If things come easy they are much less appreciated than things that you work for. Take for example; you bought a pair of Walmart jeans that were on sale for $12.99. A month later you got grass stains on them or maybe you ripped them. You think to yourself "darn, oh well I'll go get a new pair next week". Versus, you see a $200 pair of designer jeans, that you are dying to have. You pick up a few extra shifts at work to be able to afford them. When you finally have them, you are very careful to not get them dirty, and make sure to wear them on nice occasions. You know that they aren't cheap, and you recall that you had to work for them to get them. If you get a girl/guy without any efforts, compromise or work involved, it might be easy from the get go, but eventually it may be easier to let go because you didn't exert a ton of effort into the relationship.