It happens to me too sometimes. I don't think Riot is doing it on purpose. As people, we try to have a reason for everything, but sometimes things are just random like that, or we're having an off day.
One other reason could be that your MMR is right about where it's supposed to be for your skill level. So you do a little too well and it starts giving you a challenge, but you're not quite ready for that, so you lose a few and it levels itself back out. Think of it as like... you're always going to be challenged OR it's going to be easy. So use that time to challenge yourself.
Doing bad in lane or the ADC is feeding? In a way, that's a (frustrating) gift- because you can use that opportunity to learn. What can you do to compensate for your team's weakness? Can you ward more? Is it better to support a player getting more kills, or should you help someone who is falling behind by providing a gank for them? Is your team behind in gold? Can you take a tower while someone isn't looking to boost them up? There are tons of ways to improve your odds on a losing battle- just figure out what they are on a case by case basis and it'll get easier! :D
[slayer-jinx-catface]