The system of choice players on ranked?!

I play lol for 2 years only solo ranked (now i m s4) and i can t understood why i have enemies and teammates from bronze or gold ( or unraked players who dont even know what are runes) that are or very bad at the game or to good at it. I do not say that i don t pick whit players from s4 but i have matches that ar instant loses indifferent i explain or i help bronze or unranked player or i can t even defend my base other enemies being too good for me. Always when i say to myself "i want to be better at League and achieve for gold or better" i have those kinde matches and it s very frustrating.Someone can explain to me why it s like that? Thank you.