Well, such a question doesn't have an answer. That's how the language was made, and that's how it will stay. With regards to your second question, it's just a matter of "sense". However, some words have notable signs — for example, most words that end with ة (taa' marbuta) or ت (taa' maftuha) are usually feminine, but that's not a rule.

But nice topic, It reminded me of something I read long time ago, I’ll translated to English... There was an argument between a male and female He said to her: Haven’t you noticed that the “Universe” (الكون) is a male? She said: No, I’ve noticed that “Being” (الكينونة) is a female. He said: Didn’t you realize that “Light” (النور) is a male? She said: I realized that the “Sun” (الشمس) is a female. He said: Isn’t “Generosity” (الكرم) a male? She said: Yes, but “Dignity” (الكرامة) is a female. He said: Don’t you like that “Poetry” (الشعر) is a male? She said: What I like more is that “Feelings” (المشاعر) are female. He said: Do you know that “Science” (العلم) is a male? She said: I know that “Knowledge” (المعرفة) is a female. He said: I heard someone saying that “Betrayal” (الخيانة) is a female. She said: I saw some writing that “Treachery” (الغدر) is a male. He said: But they say “Deceit” (الخديعة) is a female. She said: Actually they say “Lying” (الكذب) is a male. He said: Somebody assured me that “Foolishness” (الحماقة) is a female. She said: Somebody proved me that “Idiosity” (الغباء) is a male. He said: I think that “Crime” (الجريمة) is a female. She said: And I know that “Sin” (الإثم) is male. He said: I was taught that “Ugliness” (البشاعة) is female. She said: And I understood that “Hideousness” (القبح) is a male.

Arabic is a language that is very sensitive in terms of speech; the Arabs would use as few words as possible to get the most information across and so naturally gender falls within the sensitivity of the Arabic language. As far as I know on this topic everything is masculine by default unless they contain characteristics which are seen as feminine then they are given a feminine reference. However what bases they use to determine this would probably have to be referred to experts in the Arabic language.

Then he paused for a moment and said: It seems that you were right, “Nature” (الطبيعة) is a female. She said: you were also right, “Beauty” (الجمال) is a male He said: well “happiness” (السعادة) is a female. She said: Maybe, but “Love” (الحب) is a male. He said: I admit that “Sacrifice” (التضحية) is a female. She said: And I confess that “forgiveness” (الصفح) is a male. And the argument is still on as long as the “Question” (السؤال) is a male and the “Answer” (الأجابة) is a female.

There's no actual rule for this, I think you only can know for sure if you were good at Arabic. If we -native Arabs- stumbled in situation like that, we put the word in a sentence mostly it will get clear for us when we do it, but I'll ask my professor just to make sure.