The other day I told my husband that I was really excited to see my best friend again. We live far apart and only get to see each other once a year. He responded with a hurt look, saying, “I thought I was your best friend!” To which I replied, “No, you’re my husband. I still need my girlfriend.” 
This is one of the key dynamics that can really mess up a long-term relationship. Expecting your partner to fulfill all of your emotional needs sets you both up for disappointment. Every woman needs a girlfriend and every man needs a bromance.
I’m not saying that your partner can’t be one of your best friends—just that they shouldn’t be your only best friend. You both need and deserve to have time out with other people on a regular basis. You need a different perspective; someone to bounce ideas off, and someone who will listen to you complain when (even if it’s rare) your partner’s being a jerk. 
Having a close, emotionally intimate relationship with another person helps you become a better partner in your romantic relationship. You practice listening, being supportive and nonjudgmental, and you have the opportunity to express your own ideas, visions, and dreams. All these skills are necessary for a successful romantic relationship, and they are sometimes easier to develop with someone you’re not sleeping with.

There’s another important reason to develop and foster a close, intimate relationship with someone other than your partner. When your partner is the person who fills that role in your life, eventually you lose your physical desire for them. They become too close, and familiarity breeds boredom. This dynamic is referred to as “emotional fusion.” It’s when two become one, which is a nice romantic ideal that we’re all told we should strive for in our romantic relationships. And it’s a killer of passion.
Emotional fusion occurs when the boundaries between two people become fuzzy and break down. It happens when a person feels responsible for their partner’s emotional state; if he’s happy then she’s happy, and if he’s upset then she has to fix it. I have a friend who jokes, “When I want your opinion, I’ll give it to you.” When that happens in relationships, it’s emotional fusion. It’s often very subtle, but sometimes it’s clearly obvious. 
Unfortunately, this dysfunction in a relationship is considered normal in our society. It’s even romanticized in movies and television, as in Jerry McGuire, when Tom Cruise says, “You complete me.” That’s a load of bull: if someone else “completes” you, are you really a whole person? I doubt it. 
Having a girlfriend (or a boyfriend if you’re a guy) helps you get and stay clear about who you are as a person, in and out of a relationship. Don’t ditch your friends when you fall in love—they’ll help you stay real.
 Source: Yahoo! Shine