Keep in mind that it doesn't take long to develop habits. If its your habit to be alone and get on the laptop, make a snack, maybe a big yummy snack, and mull over your lack of social life, well, then that's probably what you are going to do the next day. And the next day. You can get super used to it, so much that if you did something else you might almost miss it. Weird isn't it? Like, the thought of doing something totally random, something fun and positive you've never done, can seem like its just not an option - even though it totally IS an option. Anything, talk to a stranger at lunch, maybe they will like you or maybe not. Just getting a response will teach you something.
When you are in a time of your life when lots of people are calling, when you can't even make it to all the parties you have available, do you think you are really thinking about whether someone is going to call you or not? No, you are just living with a totally different set of mental habits and ways of interacting with people.
Feeling down really is a habit. The truth is no one will make it go away but you, because if its someone else's job to make it go away, then you won't do it on your own, and they will have to do it forever. Think about it if you had to cheer someone else up every single day. Or, instead, what if you could use that energy instead to have fun with someone who was already up?
Its how everyone feels. Everyone gets down but some people don't know how to pull themselves out of it. Sometimes its as easy as healthy habits and exercise. Once you do pull out of it, even its by chance, don't expect magic to keep you happy, you have to work a little to keep the momentum going! Even when things don't work out socially, try something new and different, watch, feel and listen to the feedback you get from the world. If you don't know what to say, just listen and take interest, ask questions, people love to talk about themselves and how great they are.