The simple explanation

Packet loss almost always comes down to something along the route between you and the server being too busy, too broken, or too weak to handle the traffic it’s being asked to move. Your packets are travelling through a chain of equipment, your router, your ISP’s network, possibly undersea cables, and then the destination’s network at the other end. If any link in that chain has a problem, packets get dropped.

The easiest way to picture it is a motorway. If the road is jammed, lanes are closed for repairs, or a junction is broken, cars don’t reach their destination on time. Some get diverted, some give up. Packets behave the same way.

The most common causes

Network congestion is the biggest cause by a wide margin. When too much traffic hits a router or switch at once, the device runs out of room in its memory buffer and starts dropping packets to cope. This is why your connection often feels worse in the evenings when everyone on your street is streaming Netflix.

Faulty or outdated hardware is the next big one. Old routers, dodgy modems, worn ethernet cables, and damaged ports all degrade the signal or fail to forward packets correctly. A router that was fine five years ago may not have the processing power to handle modern traffic loads. Loose cables and bent connectors also cause silent drops that look exactly like a congestion problem.

Weak Wi-Fi signal is a huge cause for home users specifically. Wi-Fi packets travel through the air, and walls, microwaves, neighbouring networks on the same channel, and distance from the router all interfere with that signal. The further you sit from your router, the more packets get corrupted in transit and have to be discarded.

Software bugs and misconfiguration cause a quieter but real share of packet loss. Outdated router firmware, incorrect MTU settings, badly written QoS rules, or driver issues on your computer can all drop packets that should have made it through.

The less obvious causes

Security incidents can drive sudden packet loss. A DDoS attack floods a network with junk traffic, which crowds out legitimate packets. Malware on your own device can saturate your connection from the inside. There’s also a specific type of attack called a packet drop attack, where an attacker compromises a router and instructs it to silently discard traffic.

ISP-side problems are common and frustrating because you can’t fix them yourself. A failing router at your ISP’s exchange, congestion on their backhaul, or routing issues between providers can all cause loss that looks like it’s your fault but isn’t. Long-distance traffic is also inherently more prone to loss simply because it passes through more equipment, so a call to someone in Australia will statistically lose more packets than a call to someone in the same city.

Environmental factors get overlooked. Heat causes equipment to throttle or crash. Humidity corrodes connections over time. Power fluctuations can briefly reset network gear. In data centres these conditions are tightly controlled for exactly this reason.