So many people with gender issues ...
I'm joking, but it saddens me to see that none or very few of you apparently have had a German teacher knowledgeable in linguistics. At least it does not surprise me that the natives here think gender assignments are practically arbitrary and that you just know them by being a native speaker. That's normal and understandable. You speak the language fluently, why should you care about specific domain knowledge only linguists would likely have? In teaching German as a second language, however, the system behind it should be properly explained to avoid the confusion we're seeing here.
Because the truth is, of course, that there is a simple--but very effective!--system at work here that you don't consciously realise when you grow up with the language. Gender is a way of classifying nouns into specific categories and it developed relatively quickly for very specific reasons--I can elaborate if you want me to. Anyway, for German it works as follows:
- Masculine is the default gender (with the article 'der'). All words come to end up here if they don't meet the criteria of the other categories. Words can also evolve in their usage. For example, I've noticed that, a bit more than a decade ago, Germans began using 'blog' by importing it as 'das Blog', classifying it neuter probably as part of a general misconception that words in languages that don't decline gender anymore are somehow neutral. Anyway, people who started using blogs at that time are more likely to still use 'das', whereas the general populace has since adopted it into the default gender: 'der Blog'. Both are correct usage for now, until all neuter users die off and usage will slowly normalise into masculine. Further examples: 'der Artikel' (the article), 'der Typ' (the type or the guy), 'der Partner' (the partner), 'der Teufel' (the devil), 'der Zwilling' (the twin). The list is endless, really.
- Feminine is the gender of grouping and plural ('die'). Yes, you all know that the article 'die' goes with plural nouns. What about singular usage? It's pretty easy: It's used for things that signify a group or represent a grouping of things or ideas of some kind. Things that are collectives that are thought to act singularly. Want examples? 'Die Band' (the music group/band), 'die Gruppe' (the group), 'die Schule' (school), 'die Liebe' (love), 'die Philosophie' (philosophy). I think this also portrays a bit why plurals as you now understand them--collections of singular things rather than singularly acting collectives--had later developed to use the feminine.
- Neuter is the gender of abstraction and things, or, more accurately, an action that has resulted in something ('das'). Now, you may think to yourself that this seems rather similar to the feminine usage for a collective of things, and historically speaking you'd be right, as the feminine developed last. Everything was neuter at first. What are examples, then, of abstraction or actions resulting in things? Well, some may be a bit hard to grasp. Take 'das Boot' (the boat). Clearly, it's a thing. But it is also, and more importantly from a grammatical standpoint, a thing that resulted from an action being undertaken. Other examples are 'das Los' (the lottery ticket, sometimes used metaphorically to mean one's destiny), 'das Leben' (life), 'das Tier' (the animal), 'das Gesicht' (the face).
This is not foolproof for learning, as you might imagine, as a lot of words are so old that the perfective action it represents is somewhat obscured to a modern speaker. Take 'Gesicht', for instance, which originally meant something like that which is seen by looking with your eyes (at other eyes, presumably). The prefix ge- was a way of creating perfectives for a long time in German (as much as it is used for creating the past participle of German's constructed simple past these days) plus the '-sicht' which stands for the imperfective action of seeing/looking. The act of simply looking was interpreted as having no result and thus required the perfective prefix. You see: Ad hoc explanations usually don't work in these cases and some etymology may be required in a few instances.
So there you go! The German gender system in a nutshell: If a noun is not feminine (not a collective of things), it is neuter (an abstraction or action that led to something). If it is not neuter (or not neuter anymore), it is by default masculine.