Is it normal to want a guy from work...

There is this guy at my work, I've only ever talked to him like twice, but I find him Extremely attractive. I sometimes find myself fantasizing about him, I fantasize that he walks up to me and pulls me into his arms and kisses me so passionately. I also find myself mesmerized by the sound of his voice. Every time I hear him talk it just makes me feel... I can't describe how I feel whenever I hear his voice! And whenever I see him smile I just feel so, almost weak in the knees! Idk Why I feel this way about him because I don't know him and he hardly ever even looks at me! Every time I see him and we are walking toward each other I smile at him and try to say hi but all he does is nods his head once and looks away almost instantly... Is it normal that I feel so strongly toward him even though I only see him like three or four days out of the week and I've only ever said, at most, like 30 words to him! Why is it I feel so strongly toward this guy when he practically doesn't even know of my existence! Could I maybe just grow intense feeling to quickly? Help please :/