A huge part of writing is finding someone impartial to read your work before anyone else sees it. That person needs to be your biggest critic because anything that gets passed them (a poorly constructed sentence, a whiny unlikable hero, a misspelled word) will go out to your second, third and etc. readers as well. First Readers are the people that tell you where your prose doesn't make sense, what needs more description (or less description as is sometimes the case), what needs to be clarified or expanded upon and when you have some well-beloved sentence/paragraph/etc. that is supremely unnecessary and really just needs to be cut.
No one ever just finds a perfect First Reader so it is your job to guide them into being what you need them to be. The first few times they read through a new manuscript make sure you double space and then print it out so the Reader can write comments or draw circles or stars or whatever else they might need to do to draw your attention. Train them to look for areas that are confusingly written or where they got bored (boring spots are especially good to know about). Get them to question your characters' actions and motives until they've completely torn them apart because you need to know if and where your characters stray from the core person you created them to be (also known as being Out of Character or OOC for short).
If you’re lucky, this method of reading will grow beyond just your writing and will come to affect the way your First Reader reads anything from any author. As their critical prowess expands, your writing will benefit. Just keep in mind that if your First Reader is an avid reader in general, they may come to hate you for ruining the joy of reading.
Disclaimer: Cassiopeia and cassiopeiafletcher.com have no legal liability for the hatred that may or may not result from the training of a First Reader ^-^;