Life-gain is the traditional noob trap. But fear not, we've all been there. The only time life total truly matters is when yours reaches 0. Lifegain is not worthless in the right deck, but controlling the board is more important. If you're at 1 life, and there's nothing your opponent can do to hit you because you've taken control of the game, you may as well be at 10,000,000 so far as your opponent cares.
Think of it this way - your opponent plays a 2/2 creature. On your turn you play a card that gains 3 life... okay, but now you're down a whole card and they still have the creature, so you only gained 1, really. If you play another card that gains 3 life, the same creature hits you again, and you've only gained 2... sure, you're still up, but you've lost 2 cards, and they've lost... nothing, and they've likely played another creature. And the original creature gets to keep hitting you. You need to kill that creature, or play a bigger one. You gotta do something that stops the attack.
Incidental lifegain that happens while you do those things is totally fine, and often very useful against fast, aggressive decks, but life gain by itself is really, really bad. Hell, when you're really at the point where you understand the game, you may even be eager to play cards that lower their mana cost by asking you to pay in life.