Just a few thoughts from me:
LOVE: An undefined feeling that only you can feel, sometimes you feel on top of the world with it, sometimes you feel like s**t! For sure, you DON'T need a reason to love someone coz all you want is to be with him/her all the time.
BEST FRIEND: Someone you think of, not all the time, but when you are in need, or want to share something, he/she will be the first person you wanna see/contact. Someone you trust and he/she trusts you too. You two depend your lives on one another.
SOUL MATE: This is a greater version of Best Friend. For me, a soul mate is someone whom I have the same thoughts with. Someone who ends my sentence, or someone who shares the same (dirty) jokes that nobody else can tell/get. In other words, SOUL MATES read each other's minds without even talking about it.
AS THE NEXT BEST THING: Personally, I pity those who are the NEXT BEST THING for anyone. In other words, this person is a 'second choice', which means, there is somebody else BETTER than him/her. It would really be very hurting for being the NEXT BEST THING...