Don't 'collect gear'. Figure out what you need to do that you can't with your current gear, then get what addresses that gap.
+1. This is a question that pops up repeatedly so I thought I'd chip in with a more detailed response and share my thoughts... Sometimes gear isn't needed at all, but rather knowledge. In order of importance: Knowledge/Training, Lighting, Lenses, Bodies
Photography is creating an image using light. Understanding and being able to craft light is the most important thing in photography. Skillful use of a cheap body and kit lens and "good light" will produce better results than a $6000 body with a $4000 lens in "bad light".
That being said, you should look at equipment purchases as investments. You are investing in something that can influence the quality or existence of your final images, whether that investment is in training, lighting equipment, lenses or a body.e.g. Wider aperture lenses could offer faster AF which could mean the difference between getting a shot at a critical moment vs missing the shot.
Another factor to consider is that a lens with good optics can hold its value better than a lens that is bad optically. e.g. Something like say the 100mm f/2.8 L IS will hold its value over time whereas you might not be able to sell a poor quality lens down the line to fund upgrades, because there is little demand for a weak-performance 2nd-hand lens.
The third factor I'd mention is that having more equipment leads to higher insurance premiums. So you should assess every [however often] to see what equipment is actually bringing in the money/the-shots-you-want vs just sitting in a closet leaching away your money in insurance.